Bugs

Bugs (or Glitches) are errors, flaws, or failures in a computer program that cause an incorrect or unexpected result, or behave in unintended/unexpected ways. Most bugs arise from mistakes made in either a program's code or flawed design. Bugs are usually patched quickly by developers, so a very well-known bug won't exist for long.

In Left 4 Dead, the most notorious bugs are exploited to gain achievements or Versus advantages. This page is meant to provide a comprehensive list of currently known bugs for both Left 4 Dead and Left 4 Dead 2.

Left 4 Dead

 * In the first chapter of No Mercy, when playing as the Infected in Versus Mode, there is a military vehicle protruding through the wall of the apartments on the bottom floor. If you go within a few feet of the vehicle, you die instantly for no apparent reason (there is a trigger_hurt entity in the map file, presumably to prevent survivors from falling on the humvee and reaching the below area or to prevent getting outside the map by clipping through the humvee on the other side) and Tanks will constantly take 400 damage. This also applies for Survivors. The vehicle is commonly referred to by players as "The Killer Jeep", although the vehicle is technically a Humvee.
 * If a Survivor is standing next to an object that cannot be moved, and a Tank punches them into the object, the Survivor will get stuck. The Survivors will not be able to use any guns, or items while stuck. The only actions possible are spinning in circles and turning your flashlight on and off. It is possible to be freed by being pounced by a Hunter or getting incapacitated (after being stuck for 10 seconds, you will teleport to the nearest survivor or die if no other survivors remain).
 * Near the end of The Sewer, Bots will sometimes get stuck next to the manhole, and be unable to move away from it.
 * In the final chapter of No Mercy when a Survivor is standing above the pipe near the minigun and a Smoker drags them down the pipe, they might actually be pulled above the electric generator but will be stuck between the two circles, not being choked and not taking damage. This glitch is very rare.
 * If a Tank is attacking a player in the Safe Room, and the player crouches and stays right behind the Safe Room door, the Tank will target the player and very likely won't break the door, resulting in it being stuck swinging his fist but not doing any damage to the player sitting behind the door.
 * It is possible to launch yourself from a minigun as a Survivor if you jump behind it and quickly mount and dismount it. Survivors usually won't move too far away, but if done properly, it may be enough to e.g. fall from the roof of No Mercy hospital when playing Versus/Survival version of map.
 * In the Crash Course DLC (possibly other campaigns, untested), in Versus mode, Special Infected can be killed by shooting where they are standing (during spawn period) before they have spawned.
 * In the Xbox 360 version (possibly PC version as well), it is possible to start a Versus mode game against a team entirely composed of computers. This is caused when two or more people are playing and one of them joins the Infected team and the other is a Survivor, although the Survivor cannot be the host. If the Survivor goes to the menu to change characters, the host can restart the server with the Survivor on the Infected team. This is actually still possible and serves as a great way to practice your Infected tactics such as hiding spots and Hunter wall jumps. If you wait 90 seconds, the safe room door will unbar and open, presumably implemented as a way to prevent the survivors from going AFK inside the safe room and permanently halting the match.
 * If a player is healing someone and the person being healed is constricted by a Smoker and pulled away, sometimes instead of automatically stopping the process, the game will allow the attacked teammate to be healed.
 * Sometimes if a Witch is behind a door and a player opens it, she will not be startled but instead act as if she has killed a Survivor and run off. Doesn't work if she notices the Survivor approaching and gets up.
 * Sometimes Tanks may get stuck on closed safe room doors even when the metal bar has been taken off and no Survivor is blocking him, allowing the Survivors to easily kill him.
 * If you start to reload and then pause your game, the reload will still continue. However, when you resume the game, you will have to reload again. This works best on an auto shotgun. If you use all of the shells, pause and then reload the game at the eighth shell, and it will reload the other two. Resuming the game will reload all 10 shells. However, it will still say eight shells until the ninth and 10th are entered into the gun. This glitch is only effective on the PC version.
 * On Xbox 360, you can make expert campaigns easier using the safe room exploitation. First, play through any campaign on easy until you reach the safe room before the finale. After the finale is loaded, do not exit the safe room. Call a vote to return to the lobby. Once back in the lobby, change the difficulty to expert and change the level to the finale. Once you have done this, load the game. You should start in the same safe room before you called the vote to return to the lobby. All your progress for the campaign has been instantly changed so it counts towards expert completion. However, you must complete the finale level on expert, which can still be very difficult. It is recommended to use campaign-specific finale exploits/methods to make the finale easier.
 * Rarely in Campaign mode, if your team loses while you are incapacitated, you might come back with the screen slanted, as if you were still incapacitated. This remains until the round ends. Being incapacitated again will not fix the glitch; instead, the screen will slant further. This bug can be fixed by being knocked back because your screen shakes.
 * If you startle the Witch just as a Smoker constricts you, instead of hitting you, the Witch will get rid of the Smoker's tongue first. This does not work on the Xbox versions.
 * Sometimes deadstopping a Hunter will cause him to get stuck inside a Survivor's body, leaving him very hard to shoot as most bullets will hit the Survivor instead of the Hunter.
 * In the campaign mode for Dead Air during the Runway Finale, it is possible to get on top of the airplane. If a luggage cart is positioned near the left wing of the aircraft, and there are at least three propane tanks, it is possible for a Survivor to pile the three propane tanks under the cart, explode them, flip the cart over near the edge of the wing, and jump atop of the wing. Being on top of the plane gives 100% protection from Tank punches, Boomer scratches, and the Horde. Smokers can still remain in visibility to pull Survivors off, and lured Tanks and hunters can punch and pounce them, respectively. The rear of the plane has no collision model and any entity that walks on it will fall through the aircraft, so Survivors must stay on the wings on the center of the aircraft. This has been removed for versus play as no luggage carts spawn near enough to the plane for a Survivor to flip on near the wing.
 * On the same map in Survival mode, standing near the damaged fence between the truck located near broken parts of a plane and police tape (on a small gray piece of dirt and rubbish which isn't colliding with Survivors) may cause the AI Director to completely stop spawning most Infected, even if bots are standing in other normally accessible places. The only possible threats are Smokers and Tanks, but they both have a hard time getting to hiding players, usually ending in Tanks dying out of frustration and Smokers walking around the plane and being easily killed by bots.
 * In the Death Toll Boathouse Finale (and in every other custom map with water deep enough to make your Survivor drown) you can easily heal yourself to 100 health. You need to have at least about 30 HP and start to heal when you start to lose health due to drowning damage. If done properly, you will get something above 80 from healing and after getting up you will gain additional health lost from drowning. Then, you can drop down to below 50 HP, eat pills and after getting up temporarily boost yourself up to 150 HP.
 * If you don't need to get full health back, then you can eat pills and make your Survivor drown – your health will be restored as permanent instead of temporary. Interestingly enough, if you get incapacitated twice because of drowning and get revived in black and white mode, as long as you stand up to get back your health, your vision will become normal again as if you healed up and the next incapacitation won't cause death.
 * If you heal either yourself or a teammate while the safe room door closes with all the Survivors, some of the process will be skipped and healing will be quicker.
 * However, if you do this on an idling player, the Survivor won't be healed, but the medkit will still be used. Applies to both Left 4 Dead and Left 4 Dead 2.
 * Known only to work with Hunting Rifle, it is possible to melee with your weapon and get a headshot, but this is rare.
 * There is an odd glitch that occurs when Bill is shot by Francis.
 * When the glitch occurs, one can see (as a different Survivor or in third-person view) that Francis' head and mouth move, which is in sync with Bill's words. When shooting Bill from a distance, the Francis player can hear Bill's scolding clearly, as if Francis was speaking with Bill's voice. Francis also displays Bill's unique facial animation. Using one of Francis' voice commands while shooting Bill will cause the friendly fire quotes to play properly because Francis is already playing a Response Rule.
 * An easy way to see this is by selecting Francis, aiming at Bill, then switching into third-person shoulder mode and setting "cam_idealyaw" to 100-150. Another way to check it is to use any of voice commands when Bill is yelling at Francis – it will instantly cause Bill to stop talking.
 * The glitch has to do with the Response Rules for Bill, as the FriendlyFireFrancisNamVet response is present and set to IsBiker which makes the response's rule search for Francis (who is marked as Biker in the global_actors.txt), causing Francis instead of Bill to shout. This glitch occurs in both Left 4 Dead and ported campaigns in Left 4 Dead 2.
 * When someone is playing as Zoey, has low ping and uses one of the shotguns, after loading more than one shell her arms may get stuck in the position of loading shells into the weapon. Can be fixed by changing weapons, being attacked by Special Infected (after being released the shotgun is reequipped) or reloading again. Doesn't occur when using the full reload animation. Also doesn't occur in L4D2 as Survivors there use only full reload animations (excluding pistols).
 * It is (was?) possible for Hunters to jump 'under' certain doors such as the locked alarm-door in the second level of Blood Harvest (Video) and some safe room doors. Crouch near the door and pounce at it repeatedly.
 * Very rarely, it is possible for three Tanks to spawn on the roof of the apartments at the beginning of No Mercy.
 * During a Versus match it is possible to revive incapacitated survivors as the infected by holding X near them. The camera will pan out and put the special infected in a third person camera and state "Helping up your friend" with the status bar filling up. This is possible for all playable Special Infected, even while spawning.
 * Sometimes when doing an action such as helping up a Survivor, the rescuer may look idle and not show the helping up animation. This usually happens when the use key (to help teammate) was pressed while the target Survivor was in midair. Occurs in both Left 4 Dead and Left 4 Dead 2.
 * While on the Bridge level of the Blood Harvest campaign it is possible to glitch through a wall. You will be underneath the level, only able to be harmed by extremely rare common infected spawns or resurfacing. Directly outside of the survival spawn-point, there is a red train car with supplies on it. Behind that train car is the wall through which you are able to slide. You must run into the certain sections of the displacement wall and crouch repeatedly (keyboard and mouse users can bind a key to "toggle_duck" to emulate a controller user pressing LB). It does not help if you melee repeatedly, nor if you hold a propane tank or gas can. Once you slide through the wall, you will be able to see unused blank white space off in the distance, and through the walls in which the game actually takes place. If one gets stuck midway in the displacement, not fully inside, they can go idle (assuming another player is in the game) to warp through or out of the displacement, whichever is closer. Once inside the white space, it is possible to crouch, and walk to where you start the panic event. Be careful while exploring under the world because it is possible to get yourself stuck, which will cause you to teleport to the nearest survivor after 10 seconds. If however, you are the last survivor, you will be stuck in the ground, and will die because there is nobody to teleport to. Smokers, Boomers, and Hunters will tend to avoid the spot, while Tanks will run up to the survivors, attack the ground, and then die.
 * It is very easy to create a 5th Survivor on any map in online play. You have to have at least 2 players that know how to use console commands. After the start of the game, you need to spectate and then use a console command to set yourself as a Survivor that is already taken by another player (jointeam 2 < name of Survivor > ). This will create an additional Survivor waiting for rescue in the nearest rescue closet with nothing but a single pistol – and it will be you. Possibly it allows you to create more than one additional Survivor, but it will always be a bot and nobody more can join and play as additional Survivors over the maximum number of slots. After map transitions, the number of Survivors will be set back again to four. Interestingly enough, it is possible for game to actually get rid of one of Survivors and instead replace it with a copy of other one, allowing to have e.g. 2 Zoeys in game. However, if somebody joins the game, in this case the second Zoey will be replaced with the missing Survivor. Doesn't work in L4D2 as you are not allowed to use any of commands needed to perform this glitch.
 * In Versus mode, playing No Mercy's finale and dying as the infected sometimes causes the game to crash with no error screen.
 * On the Finale of No Mercy, go into the Developer Commentary (if you have it) and then go to the minigun. Once there: jump, crouch, melee, and then press X. If done correctly, you will get teleported in/above the spawn room. (Xbox 360/One)
 * In very rare cases, while playing co-op offline on split screen, after killing a Tank another one will spawn, creating an infinite horde of Tanks. (Xbox 360/One)
 * On the PC version of the game, the credits map from the extra's menu will not display correctly. Instead, the player is free to walk around in the credits room that contains the bare minimum for a functional map. When the credits map is properly loaded, it runs the OnGameplayStart function from the director, which triggers a logic_relay that locks the camera for the survivors in this case. When just the level is changed, the OnGameplayStart function isn't called, thus the camera and survivor positions aren't locked, and the survivors spawn at the info_player_start entity instead.
 * When playing the Xbox 360 version of the game via backward compatibility (i.e. Xbox One), on rare occasions during gameplay, the audio will cut off completely and the only way to fix this is restarting the game.
 * It is possible to travel significantly faster than is ordinarily possible by using a technique known as "bunnyhopping". Players have limits to their maximum possible speed in any given direction that they are actively moving, whether that is on the ground or through the air. Additionally, when not moving in a specified direction and touching the ground, friction very quickly brings all movement in that direction to a stop. However, if the player is in the air and not holding down the movement button in the direction they are travelling, it is possible for them to accelerate without a maximum speed. To do this, players will jump and use the left and right strafe buttons without pressing the forward button, while looking from side to side, so that part of the speed they gain from moving side to side is added to their forward momentum. The frame that they hit the ground, before friction reduces their speed, the player will jump again immediately, preserving their forward momentum and allowing them to gain more by repeating the strafe movement. This allows players to achieve speeds several times faster than their standard maximum movement speed.
 * Additionally, if a player jumps immediately after hitting a downward slope, the same momentum-preserving property takes effect, but the slope converts some of the player's downward momentum from their fall into forward momentum in the direction of the slope. The player can thus gain yet more speed.

General

 * When escaping from a finale, during the cutscene, if any Survivors are incapacitated near the rescue vehicle, their bodies are not shown. Despite this, you can still hear them calling for help. However, if a Survivor is being pinned by a Special Infected, only the Special Infected can be seen on the screen.
 * If the other team votes on kicking a player, you will be able to see a vote passed and failed window. This is unpredictable, and will show random things in the window (such as "Vote passed! Kicking player - Unnamed", which will then change to the actual name).
 * It is possible to save yourself and get a message "YOU saved YOU", for example if you ignited a Special Infected with a Molotov or Incendiary Ammunition before it grabbed you.
 * An exploit allows you to earn THE REAL DEAL and STILL SOMETHING TO PROVE achievements "easily" (there is a lot of team coordination and timing involved). Each member of the team are assigned one of two roles: sleepers and glitchers. Sleepers will be receiving the credit for Expert/Realism completion, glitchers will do the work but won't get anything. The best scenario is to have two sleepers and two glitchers and then trade roles. Before starting the campaign, make sure the game is set to "private" and have everyone in party chat. Play through any campaign on easy until you reach the safe room before the finale. Once the finale safe room loads, do not leave the safe room. Sleepers quit out of the game and return to the main menu leaving the glitchers and bots in the safe room. Before leaving the safe room, one of the glitchers needs to send game invites to all the sleepers. Sleepers need to open the invite, and highlight "accept invite" but do not accept the invite yet. Glitchers can now exit the safe room and play through the finale on easy. Right before the rescue vehicle arrives (after the second Tank), a glitcher needs to call a vote and change the difficulty to Expert. The moment the vote passes, sleepers need to hit accept invite and enter the game. Once in the game, sleepers need to immediately get to the rescue vehicle. If done correctly, any sleeper who makes it to the rescue vehicle will receive the achievement.
 * The Taannkk!! Mutation has numerous issues, most notably that Tanks can spawn outside of the map, and are unable to attack the Survivors or respawn unless killed by another Tank. This seems to occur most frequently in No Mercy. In addition, it's quite common for Tanks to spawn far behind Survivors, and be unable to catch up to them, however this is most likely because the Tanks would gain a huge advantage if they spawned ahead of the Survivors.
 * A minor HUD glitch can occur if one is using third person view (via developer console). If the player heals with a First Aid Kit in first person view, but then switches to third person while healing and finishes in this state, most of HUD will disappear (only own health will remain). Can be fixed by going back to first person view or switching the map.
 * If a Hunter pounces Rochelle during the first two chapters of Dead Center, Nick says "Get that zombie off him!", despite Rochelle being a woman. Or in case of a Jockey, Nick utters "Ten bucks says he takes him to the right".
 * Captions of Nick's lines in The Passing, as well as Louis' and Zoey's in Crash Course, are wrongly colored in Francis' gray hue.
 * Pressing the Home button or opening the guide will freeze the game until you go back into the game. It may also cause the loading time to extend. [Xbox One]
 * Many exploits are associated with the "Take a Break" feature, which allows the AI to take over a player's Survivor for a period of time.
 * Taking a break and regaining control of your Survivor while being pummeled by a Charger will reset the timer between hits, meaning that as long as the player repeatedly takes a break and regains control of their Survivor while being pummeled, they will not take damage from the Charger.
 * Taking a break while covered in Boomer bile will remove the obscuring effects of the Boomer bile from your screen. You will still attract zombies and be unable to see the outlines of your teammates until the bile wears off, however.
 * It is possible to instantly get up after being charged, hunted, hit by a Tank or a Tank's rock or defibrillated by taking a break.
 * It is possible to interrupt stumbles (caused by propane tanks, oxygen tanks, explosive barrels, Hunters or Jockeys pouncing a teammate near you and Chargers charging into an obstacle near you) by taking a break.
 * If one or more Survivors are extremely far away from the rest of their team and all of them take a break, instead of attempting to walk back to their team, the bots will stand still for a few moments before teleporting to the other Survivors.
 * Left 4 Dead 2 uses the same physics engine as Left 4 Dead, which allows players to use the same technique for bunnyhopping as described in the section for Left 4 Dead.

Infected

 * Any AI-controlled Special Infected (except Smoker) can melee even if it is stumbled. This is especially annoying in case of Jockey, which instantly hits Survivor if it didn't manage to lunge onto him.
 * If you shove a Hunter as soon as he gets on another Survivor, he will not stumble. If you kill him or melee him off the Survivor, he will stumble and you will end up with two or even three "You saved VICTIM".
 * In The Barns in Dark Carnival and sometimes in The Parish, the common infected's head or arms are seen through a wall or fence, and must be shot down before disappearing. This is called the Head Of Glitch.
 * If you're being dragged by a Smoker and standing on some obstacle which a Tank can move (like a dumpster or car), you will suffer choking damage while still being dragged. Same thing can happen when a Smoker hits a jumping Survivor, although you will also see the choking Survivor getting pulled in mid-air.
 * Sometimes, if you are stuck on an obstacle with a Smoker choking you and Infected nearby are attacking you, you will not suffer the Smoker's choking damage and instead be hit from anyone attacking you.
 * When being dragged by a Smoker, if it cannot drag you any further, you may not receive any damage. Only happens in specific locations.
 * Chargers might occasionally ram players into bushes or other obstacles, causing them to get stuck briefly until the duration of their charge runs out. May even happen without an apparent obstruction.
 * If you are rammed by a Charger into a temporarily unmovable object (e.g. a healing Survivor), the Charger will stall for a second, then continue its charge backwards backwards (still carrying you).
 * If a Charger charges towards the Survivors, he will go forward, but later he may charge backwards even though he is facing the wrong way. Even happens if he grabbed a Survivor.
 * When a Charger grabs a Survivor, teammates are able to hit the grabbed Survivor with friendly fire. Survivors attacked by all other Special Infected are immune to friendly fire.
 * If you are rammed by a Charger and lose all your health when he finishes his charge, for a split second you will appear to be incapacitated as normal before being pummeled.
 * When incapacitated by a Charger indirectly, a couple of strange things can happen. You will be thrown away normally, but you will "bump" up and down while doing so. Or you will get up, but when the animation finishes, you will be instantly incapacitated.
 * If you are being pummeled by a Charger and have a Hunting Rifle or Sniper Rifle zoomed in, you will be able to view through the scope of your rifle in the direction which the camera is facing.
 * When a computer controlled Tank is pursuing you, you can crouch behind a door and the Tank won't be able to break in. He will instead be stuck swinging his fists without doing any damage.
 * If you throw a gas can close to a Wandering Witch and shoot it indirectly with the Grenade Launcher, the Witch will be set on fire, but won't be startled.
 * Occasionally, it's possible to be hit by a Tank into an out-of-bounds area and survive.
 * When Spitter spit hits a survivor, it acts as a physical object that affects players' momentum. It is theoretically possible to use that momentum to launch off of Spitter spit much faster than it is possible to run normally, although the actual execution of this trick is difficult enough that it is only done purposefully in Tool Assisted Speedruns.
 * If a Survivor is carrying a gas can, propane tank or other carried item and gets attacked by a Smoker, it is possible for them to break the Smoker's tongue by facing it and looking straight down. When they lose control of their character and start getting dragged, they will drop their carried item, spawning it directly where the Smoker's tongue is. Smoker tongues cannot exist in the same space as such items, so the tongue is automatically cut when the item spawns in on top of it. 20180901152308_1.jpg

Versus

 * If another player charges into a wall or obstacle as the Charger when you are not spawned yet, you will stumble as if you were spawned at the time.
 * Occasionally, items in a metal storage truck can be pushed through the bottom of the truck by a Jockey crouching on the item. Also, just melee-attacking it as any other Infected can cause an item to move and sometimes teleport short distances away.
 * In Scavenge mode, the Jockey can cause some gas cans to fall into the floor by standing on them and shoving them.
 * If all four Survivors are incapacitated in the ending safe room, the score will calculate twice: once because they are downed, and again because they reached the safe room. Though the Survivors won't get the 25-point survival bonus, their score is effectively doubled.
 * If some, but not all, of the Survivors are incapacitated in the ending safe room, the progress bar showing how far the incapacitated Survivors got in the level will not have a skull at the end (which it has if the Survivor dies), but the Survivor team will not receive the 25 point survival bonus for the incapacitated players.


 * In No Mercy if you run and jump into the elevator doors and spawn an infected player when its still closed between floors 8 and 5 your spawn will remain stuck in the closed doors forcing them to remain closed when the elevator gets to floor 4 the game will teleport your infected spawn somewhere else. This can only be done with no survivors nearby.
 * Occasionally, a glitch will occur where survivors get stuck in the Pummeled animation, as though a charger had grabbed them, despite not having been charged. This may be accompanied by an infected's ghost or a non-charger infected being stuck next to the pummeled survivor in a T pose. If the infected is spawned in, killing it will reset the animation and free the survivor. If the infected is a ghost, it can use the "E" button to teleport to a different survivor, teleporting its victim with it but not freeing them.

Items

 * If you deploy an ammo pack in any place you can revisit (e.g. a safe room), you can actually use it twice as long as it is not depleted.
 * It is possible to deploy ammo packs on top of movable objects, such as shopping carts in front of the Save 4 Less in The Streets, then move the object out from under the ammo pack. The ammo pack will remain suspended in the air. If the ammo pack is in the path of the infected, they will climb over it as though it is a physical obstacle, despite the Survivors being able to move through it without colliding with it.
 * When a Survivor is wielding a melee weapon, an item may appear in a fixed position between their feet.
 * If you empty the whole clip on any weapon and then swap it for a Sniper/Hunting Rifle and zoom in to the weapon you emptied the clip on and pick it up, it will switch you to your Pistol/Magnum and still be zoomed in.
 * When you are incapacitated, holding the shove button and firing your pistol will cause it not to make any noise.
 * If you hit the use button (to pick up a new weapon) on a single pistol fast enough, it is possible to equip two pistols from it (if, for example, you were wielding a melee weapon beforehand). When you equip a different secondary weapon again, you will drop both pistols, and can repeat this glitch on them to further duplicate the pistols. It is possible to generate an unlimited number of pistols this way. Note: large numbers of pistols will generate a significant amount of lag, potentially making a server unplayable if continued for an extended period of time.
 * Duplicating many pistols in this manner in a single location will cause the pistols to stack up in unusual ways; they will not fall like normal dropped weapons do, but instead form slightly offset towers or arcs.
 * If you get hit by a Tank while switching from a melee weapon to your gun, you will be stuck with your melee until hit by a Tank again.
 * If you get a Grenade Launcher and have another Survivor jump above you, you can shoot their feet with a grenade and they will fly about 50-100 ft (depends on the angle) opposite to where you're facing. This allows you to explore inaccessible areas, and in Survival, can give you an easy gold medal.
 * It is possible to do this with a Grenade too, although the Survivor won't launch as high. Getting hit with the grenade won't do any damage to either Survivor.
 * Very_visible_M60.jpg M60 is highlighted sooner than other weapons.
 * If a player was carrying an M60 when they were killed, they will be carrying another M60 with a full clip once they are released from a closet or spawn in the next map. Any special ammo will not be kept.
 * If a door closes on an Assault Rifle while it's in mid-air after being dropped by a Survivor, the Assault Rifle will cause the door to shut halfway and standing in the corner where the gun is will cause the Survivor to rotate clockwise until they step away from the door.
 * Car Alarms will not go off if shot with Incendiary Ammunition.
 * While using a Pipe bomb, it is possible to gain a "protect" and "hit" at the same time. To do this, Commons must be attacking and a Pipe bomb must be thrown within range enough for a "protect". Once the Pipe bomb explodes, you'll receive a message saying "(Player name) protected (player/bot name), but the "hit" message will not appear.
 * Chainsaw_aerial-blood_glitch.jpg visual glitch on the Chainsaw can be found if the player uses the Chainsaw for a long time against the Infected or on the Survivors, then immediately points upward on the air (without even releasing the button); this causes the stream of blood from the saw to remain whilst the button is held. It will be displayed like the player is hitting an enemy in mid-air. Blood even drops down the floor during this process. However, this stops if the Chainsaw is out of gas or if the player has released the button.
 * When interacting with weapons while holding a chainsaw, it the captions will show the [chainsaw starts] text, even though you're not switching to your chainsaw.
 * Awkward_arm_position.jpg melee weapons are glitched when playing in third person.
 * There are no animation for one of the swings for the Guitar.
 * When using buttons or opening doors (usually done with the Survivor's left hand), a Survivor will use their right hand, but in the position of their left hand, resulting in an awkward animation. This occurs with the Katana, Baseball Bat, Cricket Bat and Golf Club. This was fixed; Survivors now use their left hand to push buttons, but it makes their weapon flicker.
 * While on a local server, switching to third person mode can cause your melee weapon to become invisible. The weapon can still be used properly, although you do suffer from character model issues, e.g. your arm goes the wrong direction and inside your body when you pick up a weapon.

Levels

 * In The Mall, at the escalator from the second to the third floor during the crescendo event, if a smoker pulls a survivor off the edge of the escalator, they will not hang from the edge as expected when pulling survivors off fatal drops. Instead, they will fall partway to the ground, then hang suspended by the smoker being strangled until they are freed. Depending on how far they made it up the escalator, they will then drop a very short distance to the ground and either be killed instantly (despite any health they had left) or be incapacitated.
 * In any finale, if the last player standing (i.e. not incapacitated or dead) dies on the rescue vehicle (e.g. falling to their death on top of it), or if an incapacitated player is on the rescue vehicle when the rest of the team dies, the escape cutscene for that finale will begin to play as the music cue for the Survivors dying starts, before returning the Survivors to the safe room.
 * In the Atrium, because this cutscene partially occurs in slow-motion, the Survivors will be in slow-motion once they are respawned in the safe room.
 * In the Atrium, before the elevator, if you go to the plant near the elevator and jump to the edge, your character will make their falling to death sound and die.
 * In the Atrium, by starting the elevator when someone is incapacitated outside of the elevator, it is possible to begin the finale by pressing the use key on the Information stand near the car. Though the finale has been started, it will behave strangely. When the finale is started this way, any gas cans put into the car in the first 5-10 seconds of the finale will not count, possibly causing the finale to continue even after all of the gas cans have been used.
 * In the Atrium, AI-controlled Survivors may accidentally kill themselves by jumping from the 3rd floor bridge. This occurs under specific conditions. First, the glass between the 3rd floor bridge and the stairs leading up to the 3rd floor must be broken. Second, the AI-controlled Survivor must attempt to jump from the bridge to the stairs in order to reach the human-controlled Survivors. Third, a human-controlled Survivor must move from the top of the stairs to the bridge while the AI-controlled Survivor is in the middle of its jump. The AI-controlled Survivor will attempt to abort its jump and return to the bridge, fail, and fall to its death.
 * In the Underground, the Survivors go down a long set of stairs and then drop into the sewers. Around the back of where they fell, there's a broken set of stairs. If you attempt to jump up the stairs, the Survivors will verbalize as if they were falling down a steep hill.
 * During the Highway and The Park, the sound of rain may play, despite neither of the chapters having rain. [Xbox 360]
 * In the Highway, after you leave the motel, when you slide down the hill, land on the slope to the left. If you jump back onto the slope your Survivor will play another quote as though they were sliding down the hill.
 * In the Highway, after leaving the motel and sliding down the hill, go around to the other side of the big rock to the right. Next, walk towards the rock and by continuously pressing LB on the Xbox 360 controller, in certain specific spots you will clip inside the rock. Another place this can be done is right after you slide down the hill there is a rock wall to the left. This one is much harder to do, but on a specific spot, you can clip through the wall and get inside, going out of bounds and under the map. Sometimes you will get stuck while clipping through the wall, causing you to teleport to the nearest Survivor.
 * In the Highway, when at the stairs on the side of the motel closest to the bridges that the Survivors go under, if you jump on the railing on the right side of the stairs and wedge yourself between them and the floor above, your character will be considered 'stuck' by the game and, after a few seconds, will be teleported to another survivor.
 * During the Concert finale, it is possible that the Director may not spawn any Tanks or spawn only one. You can slightly increase chance for this bug to occur if you will crouch on the top of higher scaffolding (the one of left side of stage).
 * It is also possible for Tanks to spawn inside of walls. In this case, they will register as stuck and simply die after a few seconds, and no replacement Tank will spawn.
 * Becoming increasingly frequent in the Concert, the final horde and a Tank will spawn as soon as you start the tape, and the rescue helicopter will instantly spawn at one of the two stationary points for pick-up. This has also been reported to happen during the Plantation finale, where the gate will be broken down as soon as you call for rescue from Virgil. This can also occur in Town Escape finale where after you signal Virgil with the Burger Tank sign the boat will almost immediately pull up to your rescue. It also happens during the the Atrium where the final horde and endless Tanks show up once you exit the elevator. Even if you gather all gas cans and gather the Survivors by the car, you can't complete the finale, and this bug will recur if all the Survivors are killed, forcing you to return to the lobby and restart the campaign, and rarely, in No Mercy where after you call the pilot, the Skin on Our Teeth will cue then the helicopter instantly spawns.
 * In the Concert, any incapacitated Survivor who didn't enter the finale area and aren't near or in the safe room when someone else starts the finale will be removed from the map (into the main menu with a message "An error occurred with your connection"). This will permanently remove that Survivor from the game, and no other players or AI-controlled Survivors will be able to fill that slot. If a vote is made to change the campaign, the Survivor slot will still be gone in the new campaign.
 * During the Plantation, when the Survivors escape on Virgil's boat, there is an invisible corner where anything that goes there will be instantly killed. When Common Infected reach this corner, they instantly die and float in the water.
 * During the Plantation, wedging yourself under the stairs from the ground to the ground floor of the Plantation House on the far right side of the finale area will register the player as 'stuck' and, after a few seconds, you will be teleported to the nearest Survivor.
 * In the Milltown, the bots will grab the Tier 2 weapons in the safe room of Return To Town (right after the small trailer). They walk up to the wall and grab the weapon right through the wall, and can also grab any other item they wish. Human players cannot do this, even if they use the "break" option to release the control to the bots.
 * In the Sugar Mill, in Versus mode, you may be spawned as a Tank under the cane field.
 * In the Bus Depot Survival, it is possible to get on top of one of the buildings to avoid the Infected. When you start the event and turn around to go to the fence adjacent to the starting point, there is a corner with trash bags and three pipes on the wall. Continuously jump in the corner until you are on the object. Shimmy over and jump onto one of the pipes, then the next, the next, and then to the roof.
 * In the Bridge Survival, you can make the game freeze (requires a friend). Begin by taking a grenade launcher and crouching on the top-right corner of the helicopter platform. You can grenade jump your friend onto the bridge; he will run at you and jump on top of you while you shoot the grenade launcher, and he will continue running in mid-air. (This is extremely difficult, and if you're unlucky, both of you will die, him pushing you off and jumping into the water) When he is on the bridge, you can crouch under the trailer and use the teleport spot to join him. Once on the bridge, you can run all the way to the end, where the bridge's gate isn't lowered. When you walk right up to it, the game will freeze, forcing you to restart.
 * In the Bridge, there is a way to get beyond the level boundaries (video). After you reach the Tank (kill him, or else he will be a bother to you), when you get to the part after the Tank where you have to climb on top of the bus to get to the higher level of the bridge, there should be another blue bus next to that one you climbed on. Jump on that, then jump to the suspenders along the side, and walk up, you'll be on top of the bridge. Now you can just keep moving forward, going outside the map. Once you see those square buildings outside the map, there is one that you can reach from the top of the bridge (don't jump off, just fall off the edge, it does a lot less fall damage to you).
 * Starting safe rooms in the Town and the Terminal don't use proper color correction.
 * In the Port Versus or Scavenge, being vomited on by a Boomer does not always spawn a horde, rendering the Boomer useless (typically, being "biled on" spawns additional Infected at the nearest spawn site).
 * In the Port Survival, you can finish the campaign, even though you shouldn't be able to. Make your way to the ladder part of the bridge (prop-building, low gravity or grenade launching), make sure everyone is on this ledge, then hit the switch. The finale will resume like in the real campaign, the switch will turn and the bridge will raise, once the bridge stops, get down carefully, and restart the generator. Four Tanks will spawn, and an Infected ladder will be created for them to climb on the bridge; this can be disastrous if you are not prepared, as all the Tanks already here and the new ones will just rush up.
 * This will also generate a new glitch on "Survival Campaign" win (by restarting the generator), if Bill is the one who restart the generator, everyone will be awarded the KILL BILL achievement. It will also create a glitchy endgame board that can't be skipped, and will show empty stats, except for the time that will be the Survival timer at the time the generator is restarted.
 * In Motel Survival, it is also possible to finish the level by using the Grenade Launcher trick described above to launch a survivor over the invisible walls around the play area. The player can then navigate to the safe room, and if all other survivors commit suicide, the level will end and The Fairgrounds will begin. The game mode will still be considered "Survival Mode", however, so no infected or items will spawn in this new game mode, nor will any items spawn in the safe room.
 * It is also possible to start the Survival round from outside of the play area by shooting the alarmed car in front of the safe room. This will start the Survival timer and allow infected to spawn.
 * By completing The Highway and The Fairgrounds in Survival Mode, you can reach The Coaster, which actually has items and equipment available for the Survivors to use. The round can be started by activating the roller coaster, allowing infected to spawn for the Survivors to fight.
 * It is possible to get an indefinitely high time on this map by dropping down to stand on the cross beams holding the coaster up. Infected will not be able to reach you, except for The Spitter and The Smoker, making it easy to survive for a long time.
 * In every Left 4 Dead 1 Survival map ported to Left 4 Dead 2, picking up a new primary weapon will not drop your previous primary weapon. Instead, it will vanish permanently. Switching weapons enough times can completely deplete weapon spawns, making it impossible to select certain weapons until the level is restarted.
 * In The Rooftop survival map, running around the outermost part of the map will cause some (but not all) infected to get stuck in the middle, unable to pathfind to the players.
 * This will make the game believe that the players have reached an out of bounds area, causing it to spawn Spitter acid periodically to hurt them.
 * Some in bounds areas in Survival maps will register as out of bounds to the game and cause Spitter acid to periodically spawn on any survivors standing in those areas.
 * The top of the small tower behind the room with the radio in The Rooftop
 * The top of the tents closer to the helicopter on the off ramp in The Bridge
 * The piece of elevated land past the Jazz Club in the construction area in Bedlam

Fixed
For a comprehensive list of things that have been fixed or added to the game, see Update info. Some bugs need confirmation as to whether or not they are still present. Discuss on the talk page.
 * You are able to fire the entire magazine of an Assault Rifle or Submachine Gun in a single shot. [fixed 9/22]
 * A glitch involving a gas can can give you infinite Molotovs. Can be very useful in a Crescendo Event. [fixed in L4D2]
 * You're able to claw a Survivor while still selecting a spawn point, and they're completely helpless. [fixed 9/22]
 * If the Survivors see ammo piles, they may refer to it as weapons. [fixed on 1/18]
 * While using the Hospital's elevator, if you jumped enough or throw an oxygen tank at anyone, you could fall through the floor to your death. This has been prevented by not letting you jump at all.
 * If a Survivor was revived once the elevator had been activated, they were left behind. The elevator would rise but they would not. They are unable to leave the elevator shaft, and have been easy prey to the Infected.
 * A Common Infected can become invincible, which means it can't be killed but will still try to kill you. To get rid of the glitched Infected, trap it inside a place and close the door.
 * If you throw a propane tank at a Witch and shoot it, the Witch will disappear, but you can still hear her sobbing.
 * Cr0wning a witch can occasionally render her immortal. [fixed 12/1]
 * Switching weapons while using your melee attack would allow you to attack faster than normal.
 * If you tossed either a Pipe bomb or a Molotov, then immediately picked up another, you could continuously spawn new grenades.
 * After being in the close vicinity of an explosion, rapidly changing weapons would cause the player to reset the stumble animation and move him backwards, or "air walk", until the player collides with a wall or if weapon cycling ceases. This allows entry to normally unreachable areas because this cycle maintains your height, suspending you in the air if done off a ledge.
 * There was a way to have more than four Survivors. The second you open the door to the rescue closet, start to revive the dead Survivor with a defibrillator. But now when you save the Survivor in the closet, their corpse disappears.
 * Shoving the Minigun repeatedly and jumping onto the it would launch the player into the air. You can skip decent-sized chunks of maps, as well as get to out-of-reach places.
 * In The Streets, it was possible to skip the Crescendo Event by pouring a gas can into Whitaker's door instead of the cola.
 * It is possible to get past the downed airplane in Swamp Fever without ever triggering the alarm. If you walk the length of the plane to the back rows of seats there is a large gash on the right side. On the far side of the gash, crouch and walk towards the hull you will be partially outside the plane. If you stay crouched and jump, you will reach the roof of the plane and from there able to jump down without the crescendo event starting.
 * During The Passing Survival, there was a way to refill the M60's ammo. If you picked up a weapon close to M60 spawn point, it would go back to its spawn position. Upon being picked again, the M60 would be refilled with 150 bullets, regardless of how much you used. [fixed on 6/9]
 * If Nick died, Coach and Ellis would be unable to say anything until Nick was defibrillated or rescued from a closet.
 * You could defibrillate a Survivor regardless of their Z axis position (their height). You could revive a dead Survivor 100ft above you, but you have to be right next to them in other dimensions.
 * When playing with the original Left 4 Dead cast, they make one of the Left 4 Dead 2 survivors' screams when falling to death. [Fixed on P.C]
 * While you are climbing on a ladder, no Common or Special Infected can go up or down it.
 * In the Hotel, if a Survivor dies in an elevator, the elevator will descend but the Survivor will not. If the elevator starts moving and the Survivor is revived while ascending, the Survivor may respawn on top of the elevator. However, the game will quickly take notice, and will teleport you back to your team.