Bugs

Bugs (or Glitches, as they are sometimes known) are an unwanted result of a coding or programming error in a video game. There used to be many bugs in Left 4 Dead that were used to gain achievements or an advantage in Versus, or used just for fun. Bugs are normally fixed by a patch downloaded from the developers, so normally a very well-known bug won't exist for long. This page is meant to provide a comprehensive list of currently and previously known bugs for both Left 4 Dead and Left 4 Dead 2.

Left 4 Dead

 * In the Hospital chapter of No Mercy, throwing an Oxygen Tank at the other survivors on the elevator when it is moving will cause them to fall through the floor to their deaths. Works also with other throwables (also Molotovs and pipe bombs). This has been patched.
 * 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 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 (you will teleport back to the other survivors after a set amount of time).
 * Near the end of The Sewer, the AI will sometimes get stuck next to the manhole, and be unable to move away from it.
 * On occasion, if you touch an object previously punched by a Tank, you may be killed (may be due to the fact the object is moving slightly. Even if they're moving noticeably slow, objects can still incap Survivors).
 * At the final chapter of No Mercy (Rooftop Finale) when Survivor is standing above the pipe near the minigun and Smoker drags him/her down the pipe he/she 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 players in the Co-op mode, and they hide in the Safe Room, and one of the crouches and stays right behind the Safe Room door, the Tank won't be able to break the door (though he usually can do that), and will be stuck beyond it, swinging his fist, but not doing any damage to the player sitting behind the door, thus allowing to be easily killed by shooting through the door.
 * "Crouch Running" — It is possible to crouch and go running speed as both Infected or Survivor.
 * A strange turret glitch with the new campaign Crash Course. This occurs when you stand on top of the machine gun turret and operate it. This bug is commonly referred to by players as "Doing the Monkey". In L4D2 version this was partially fixed, but glitch can still be seen if there is gas can/propane tank close enough to minigun. Jumping on it and using turret will result in reproducing this glitch. Bug won't work if Survivor will set propane tank by himself as he can stand only on those which weren't moved nor picked up yet.
 * 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. You will only be able to spawn at the beginning of a campaign (No Mercy's "The Apartments," for example), as the AI do not open Safe Room doors, most likely to avoid irritating players. UPDATE: If you wait long enough, the doors will open by themselves and be unable to close, possibly due to problems with Survivor teams not opening the door, and then leaving, causing Infected players to be stuck in spawn mode (this has been tested on PC and Xbox 360).
 * If a player is healing someone and the person being healed is constricted by a Smoker and pulled away, a glitch will allow the person to be healed while dragged.
 * If a Witch is on the other side of 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.
 * 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, it will reload the other two. Resuming the game will cause it to 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 be very difficult. It is recommended to use campaign-specific finale exploits/methods to make the finale easier. Recommendations are listed below in order of increasing difficulty:
 * 1) Death Toll, 2) No Mercy, 3) Dead Air, 4) Blood Harvest <--- This is by far the hardest of all 4 campaigns to complete
 * Rarely in Campaign mode if your team loses while you are incapped, you might come back with the screen slanted, as if you were still incapped. This remains until the round ends, even getting incapped will not help; this just makes the slant twice as bad. This bug can be fixed by being knocked back, because your screen shakes, thus making your screen normal.
 * If you startle the Witch just as a Smoker constricts you, instead of killing you, the Witch will get rid of the Smoker's tongue. It will not work on the 360.
 * In the campaign mode for Dead Air during the 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 Tanks, Boomers, and the Horde. Smokers can still pull Survivors off, and Hunters can pounce them. The rear of the plane has no planning and any Survivors that walk 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.
 * Known only to work with Hunting Rifle, other weapons untested. It is possible to get a headshot by meeling with your weapon, though rare.
 * There is an odd glitch that sometimes 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 displayed Bill's unique facial animation. Using one of Francis's 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 something around 100 to 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.
 * It is (was?) possible for Hunters to jump 'under' certain doors such as the locked alarm-door in BH2 (Video) and some safe-room doors. Crouch near the door and pounce at it repeatedly. Edit: It is possible to jump through saferoom doors. Unsure of alarm doors.
 * 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 incapped survivors as the infected. Simply incapacitate them, approach them, and hold X. 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. It is possible to do as every playable Special Infected. It is also possible to do this in ghost/spawn mode. This glitch does not work on Left 4 Dead 2.
 * While in a Survival match on the Bridge level of the Blood Harvest campaign it is possible to glitch yourself through a wall. You will be outside of the level, and underneath of it, unable to be harmed. To do this you must find a specific spot. Directly out side of the spawn, there is a red train car with supplies on it, behind that train car is the wall through which you are able to slide through. You must run into the wall, and crouch repeatedly. It also helps if you melee the wall while crouching up and down, and running back and forth. It is possible, but not confirmed, that holding a propane tank, or gas canister. Once you slide through the wall, you be able to see unused blank white space off in the distance, and through the walls in which the game actually takes place. Once inside the white space, it is possible to crouch, and walk to where you start the event. Be careful while exploring under the world because it is possible to get your self stuck, and then teleport out of the white area to the nearest teammate (AI or other wise). If however, you are the last survivor, you will be stuck in the ground, and will die from the infected. Smokers, Boomers, and Hunters will tend to avoid the spot, while Tanks will run up to the survivors, attack the ground, and then die. Every now and then a single common infected will glitch through wall, and attack you.


 * In Versus mode, playing No Mercy's finale level and dying as the infected sometimes causes the game to crash with no error screen.

Left 4 Dead 2
* Some melee weapons are glitched when playing in third person mode - either there are no animation for one of swings (in case of i.e. Guitar) or, when using buttons or opening doors (where Survivor always uses left hand), he/she will use right hand, but in position of left hand, resulting in really awkward appearance. The latter bug applies for weapons, which - from view from other Survivors - requires to use both hands, so it is: Katana, Baseball Bat, Cricket Bat and Golf Club. Axe and Guitar, however, aren't glitched since they use other animations. This was somehow fixed as Survivors now just use left arm to push buttons, but it makes holding weapon flicker in very odd way.
 * If you deploy an ammo pack before you end a Chapter in a safe room or anywhere you can revisit, you can actually get two lots out of it as long as it is still there, so three people can take from it, but it must still be there. This can be useful on finales with the safe room close to the actual Crescendo Event in case you need some extra firepower. Note that this can only be done if one or three people collect the ammo, as when all players have taken the extra upgrade, the ammo pack will disappear.
 * Just like in Left 4 Dead, when a Tank is attacking in Coop mode, the players can hide in the Safe Room or any door (as long as a infected does not break it) and if one of them crouches right behind the door, the Tank won't be able to break in, but will instead be stuck behind it swinging his fists without doing any damage either to the door or to the person sitting behind it, thus rendering himself an easy target.
 * Many players have reported unexpected crashing as a result of loading or connecting to a server in spite of the most recent Left 4 Dead 2 patch fix. This may include an abrupt exit to the desktop without showing an error message, or errors such as a C++ runtime library error. A fix has not yet been released, but it is suspected to be due to inefficiencies within the Source engine or older versions of Vista (prior to the Service Packs 1 and 2), and not due to hardware limitations.
 * In Single Player/Campaign mode, any Special Infected (except of Smoker) can hit Survivors 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 and Hunter, which has the most damaging claw attack and in almost all cases attempting to kill Hunter with melee weapon results in unexpected losing plenty of health. Oddly enough, constant shoving SI is more productive as any Special Infected (expect of Charger) dies after 4 to 5 melee shoves and this tactic minimizes chance of being hit by SI.
 * Chargers might occasionally ram players into bushes or other obstacles, causing them to get stuck indefinitely until they are killed. This may even happen on solid ground.
 * In Single Player/Campaign, if a Survivor is attacked and dragged by Smoker, while standing on some obstacle which Tank can move (like trash container or car) he (or she) will be instantly choked and Survivor will suffer choking damage even that he is still dragged and shouldn't take any damage. Same thing can happen when Smoker hits jumping Survivor. In this case Survivor will not only be instantly choked, but also you will see choking Survivor (meaning game will display choking animation on him/her) getting pulled to Smoker in mid-air.
 * In Versus mode, if another player charges past you as the Charger when you are not spawned yet, you will stumble as if you were spawned at the time.
 * If a Survivor dies while in an elevator, such as the one in the first chapter of Dead Center, 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 spawn you with your team shortly.
 * It is possible for four to five of the same Special Infected to spawn at the same time in Campaign mode. This is currently unconfirmed for the Witch.
 * 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 the 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 Atrium Finale where the final horde and endless Tanks show up once you exit the elevator. Even if you gather all 13 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.
 * Also on The Concert, it is possible that during finale Director won't 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).
 * If you are rammed by a Charger into a temporarily unmovable object (e.g. a Survivor who is healing), the Charger will stall for a second, then run backwards, still carrying you, until a medium distance away from the group of Survivors. It will then pummel you normally.
 * If during the Atrium Finale, once the car is full, if the Survivors fail to escape and at least one Survivor is incapped on the podium with the car, the game will show the beginning of the escape scene while playing the death music; the scene will stop once it reaches the slow-motion part. After a short while, the game will reload the safe room, except the game will still be in slow motion.
 * If you create your own server in the Atrium Finale in Dead Center and skip the elevator by noclipping, it is possible to start the Finale by pressing the use key on the Information stand near the car. Though the Finale has been started, it will behave strangely. Sometimes when you get all the cans into the car, you will not be able to complete the level; other times, it will behave as normal (confirmed on PC and XBOX using a different method). It is also possible to do this on the Xbox by starting the elevator when someone is incapped outside of the elevator.
 * During the Bus Depot level of The Parish while playing Survival mode, it is possible to get on top of one of the buildings to avoid Common 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 Swamp Fever's finale, The Plantation, when the Survivors escape on Virgil's boat, there is an invisible corner where anything that goes there will be instantly killed. Common Infected point out this location, and when they reach that corner, they instantly die and float in the water.
 * If Charger grabs one of Survivors and his teammates are attempting to kill Charger, they will usually hit grabbed Survivor, causing friendly fire, which on harder difficulties may cause more damage even that Charger. Since Survivors attacked by other Special Infected are immune to friendly fire, it may be considered as glitch.
 * When escaping from a finale, if any Survivors are incapacitated (as opposed to pinned by Special Infected), 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.
 * The Charger does a tiny bit of damage to anyone who is near the Charger while it is charging. It is possible to be incapacitated by this, and a couple of strange things can happen:
 * You will be thrown away normally, but you will "bump" up and down while doing so.
 * You will get up, but when the animation finishes, you will be instantly incapacitated.
 * A very rare glitch may occur when a firearm appears in a fixed position between the feet of the Survivor. This may happen when a Survivor was wielding a melee weapon. The firearm cannot be picked up (confirmed on PC).
 * If you throw gas can close to Wandering Witch and shoot it indirectly with Grenade Launcher, Witch will be set on fire, but you won't startle her.
 * While you are climbing on a ladder, no special infected or regular infected can go up or down it.
 * 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. Accomplished with the Oxygen tank and the Defibrillator (unconfirmed on 360).
 * On the PC, while in a local server, switching to third-person shoulder 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.
 * While switching weapons, it is possible to have your gun completely reloaded, even if there is a different gun in a weapons area. This, however, must depend on the type and tier of the gun. For example, it is possible to gain ammo instantly between two Submachine guns (silenced and non-silenced), three assault rifles (AK-47, M16, and Combat Rifle), the two shotguns (Chrome and Pump), and the other two shotguns (Combat and Tactical). This does not work with the scoped rifles (Hunting/Sniper), however.
 * There is a very strange glitch where if you get hit by a Tank while changing from your melee weapon to your gun, you will be stuck with your melee weapon until hit by a Tank again (confirmed with chainsaw and golf club).
 * If someone else on 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, then changes to the actual name of the player on the other team).
 * 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. The hit survivor will also say something in their "friendly fire" quotes.
 * Also, hitting multiple Survivors will only result in one of them yelling back at you.
 * It is possible for a player who has thrown a Molotov to save themselves. First, a Special Infected that causes the "You saved (player/bot name)" message to appear. Burn the Special up, and make it pummel/choke/pin you. Once the Special dies to the flames, a message will appear saying that you "protected" yourself.
 * On the 360, if you are being pummeled by the Charger whilst you have a Hunting Rifle equipped, if you zoom in, you will be able to view through the scope of your rifle in the direction which the camera is facing. This has been confirmed and will work on PC. (unconfirmed with sniper rifle)
 * In Survival Mode, if you get a Grenade Launcher and have another person jump on a ledge above you, you can shoot on their feet with a grenade and they will fly about 50-100 feet to an inaccessible area, giving you a chance to earn you an easy gold medal (Confirmed on 360 and PC).
 * It is possible to make it with Boomer bile too, although Survivor won't "launch" as high as when hit with Grenade Launcher. It is also too valuable item to use in that way, but getting hit with bile doesn't make any damage to launched Survivor.
 * Similar to the first game, there is a safe room exploitation in Left 4 Dead 2 that allows you to complete expert/realism campaigns easily on Xbox 360. This exploit requires human players because there is a lot of team coordination and timing involved. Unlike L4D1, not everyone receives expert or realism credit for completing the campaign using this exploit. Each member of the team will need to be assigned one of two roles before starting the campaign: Sleepers and Glitchers. Sleepers will be receiving the credit for expert/realism completion. Glitchers actually perform the glitch but will not receive credit. 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. Like L4D1, 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 receives credit for completing the campaign on expert (and realism, if enabled). NOTE: This glitch is listed here because, by definition, it is a glitch. Once you have unlocked any achievements, they can't be undone. This exploit in not recommended if you do not wish to be deprived the experience of receiving "THE REAL DEAL" and "STILL SOMETHING TO PROVE" legitimately.
 * While playing versus and/or scavenge on The Passing: The Port, it has been observed that being vomited on by a Boomer does not always spawn a horde. Typically, being "biled on" spawns additional Infected at the nearest spawn site, the sole reason a Boomer is combat-effective. In some cases, it has been observed to attract no additional Infected at all, rendering the Boomer useless. This is not known to be observed on any other level.
 * Any player who downloaded The Passing or is playing with a player who downloaded The Passing, during any finale, Survivors may yell out either something to do with the bridge or Jimmy Gibbs Jr.'s car, regardless of the rescue vehicle. Although the Survivors can yell out the correct rescue vehicle, most of the time, they do not. This has been fixed on the PC (tested on both Xbox and PC).
 * Very_visible_M60.jpg M60 is weapon exclusive for The Passing campaign (in L4D2 campaigns) and probably to this cause it is highlighted much sooner than other weapons, which may lead to quite hilarious results.
 * Rarely in Hard Rain, you can spawn as a Tank under the cane field. Unfortunately, you cannot attack the Survivors, because you will be too far down (not tested on other campaigns or PC).
 * If you have full captions turned on when interacting with weapons while holding a chainsaw, it will show the [Chainsaw starts] caption, even though you're not switching to your chainsaw.
 * A silenced pistol glitch can be done while incapacitated. If you are down, hold the shove button and fire off your pistol, this will cause the pistols not to make any noise whatsoever.
 * In Hard Rain, when playing with bots in Milltown, they 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 they can also grab pain pills and adrenaline shots. You cannot do this as a human player.
 * You can damage yourself by hitting the green trashcans (like the ones right outside the safe room in the third stage of The Passing) with a melee weapon. For some reason, when the trashcan touches you when it goes flying when you hit it, it'll hurt. It will only do 1 damage, though. Not sure if you can hurt others by hitting them with the trashcan.
 * You can freeze the game during survival mode in The Parish's finale the Bridge. (You must have 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, making you have to restart.
 * On the final chapter of The Parish's bridge, there is a way to get out of the map. After you reach the Tank (kill him, or else he will be a bother to you), then, 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 (the very top). Now you can just keep moving forward, and 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), now fall off the top of the building and explore! This Glitch can be seen Here! (Confirmed on Xbox 360)
 * You can play on a hidden Survival level by going to the Motel on Survival, getting a grenade launcher and blasting somebody over the invisible wall by the fence. Then, they heal using a kit and/or pills. If they land on the concrete and not the grass, they will be alive with a little bit of health. They can then go through the level and to the safe room. They can also start the game by shooting the car outside of the safe room. Once everybody else has died, they will continue through the campaign until they get to the part before the rollercoaster and restart the round; they will appear in the coaster area, as the fence is blocked off. The coaster is completely supplied and set up like a survival map.
 * There is a glitch where 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 (tested on Xbox 360).
 * 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 (this has not been tried with any other gun).
 * On The Sacrifice and No Mercy for Left 4 Dead 2, the original Survivors use the falling/death screams for the sequel Survivors. This creates some humorous situations where you can be playing as Francis, and hear Rochelle! (Untested on PC)
 * In Versus or Realism Versus, if the Survivors are incapacitated in the safe room, the score will calculate twice. Once because all four Survivors are downed, and again because they reached the safe room. Though the Survivors do not receive the 25-point survival bonus, their score is effectively doubled.
 * Car alarms will not go off if shot with Incendiary Ammunition, even if the car is shot at point-blank range. This allows players to be less careful around car alarms if they have fire bullets loaded.
 * The Taannkk!! Mutation has numerous bugs and 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.
 * On the first level of Dark Carnival, after you leave the motel, when you slide down the hill (before going up the next hill to the safe room), go around to the other side of the big rock to the right. Next, walk toward the walk and continuously press LB on the Xbox 360 controller, this will place you inside of the rock.
 * On The Port Survival of The Sacrifice, 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.
 * The 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.
 * On the above, this will 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 Kill Bill. 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. You will need to rejoin to get out of it.
 * It is possible, through co-operation, to survive nigh-indefinitely in the Motel on Survival. First, get three players rocketed over the fence to the secret zone with the grenade launcher, taking healing into your own hands. Then, go to the staircase near the Motel lobby. Hop on the railing, crouch and walk up the railing to the top, then walk around a little until you are unable to move. In a few seconds, you will teleport to your teammates. Heal or revive people if you need to do so, and head to the end of the map. Do not close the saferoom door if all players or inside, or all players will be kicked, and the game will say that DLC was selected. Have one player go inside, shut the door and open it so that it faces outwards, then shoot the alarm car to start the timer. If all of the players hide in the back corner, on the door's side but away from it, some common infected will occasionally get in, Tanks will attack the door and then die for seemingly no reason and no other Special Infected will reach you.
 * As a extension to the above glitch, there is a shopping cart near the safehouse door. Until the round is started, it will be impossible to move it. After it starts, shoving the cart until the wheels face towards the sky, right next to the thin side of the door (a difficult task) will allow players to jump onto the cart, then the door, then the safehouse. There, they can either jump down into an small, enclosed area leading to an invisible wall in front of Whispering Oaks, or run across the green barrier to the larger enclosed area, although this is accessible to Tanks and Common Infected.
 * If player is carrying an M60 when they are killed, they will be carrying another M60 with a full clip, despite how much ammo was left, once they are released from a closet or the next map has loaded. Any special ammo that was loaded will not be kept.
 * A minor HUD glitch can occur if one is using 3rd person view (whether by the third person or third person`s shoulder console command). If the player heals with a First Aid Kit in first person view, but then switches to third person view while healing and finishes in this state, the player's cross hairs, items, and weapons display will become invisible. The HUD returns to normal if the player either switches back to first person view or loads the next map.
 * 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.

Fixed bugs
The following is a list of bugs that have been mentioned as being in the game, but need confirmation as to whether or not they are still in the game as of the most recent patch. These can be discussed on the talk page.
 * Cr0wning a witch (even without incendiary ammo) can occasionally render her invulnerable (extremely rare). Fixed on 12/1 update. (PC)
 * Power Shot: you are able to fire the entire magazine of an Assault Rifle or Submachine Gun in one second. Fixed on 9/22 update.
 * "Molotov replication" ― A Molotov glitch involving a gas can. This can give you infinite Molotov's and can be very useful in a Crescendo Event or a finale. Fixed on 9/22 update―Appears to be usable on Xbox 360.
 * Sometimes a Common Infected can be glitched and will become invincible, which means it cannot be killed and it will continue to attack you. To get rid of the glitched Infected, trap it inside a place and close the door so it can't get out. Melee attacks are useful at this.
 * After the battle with the horde at the elevator in The Hospital level, if a Survivor managed to become incapacitated inside the elevator for any reason and was revived once the elevator had been activated, they were left behind. The elevator would rise but they would not. They would also have been unable to leave the elevator shaft, and have been easy prey to any other Infected.
 * Also in The Hospital's elevator, if you jumped enough, you could fall through the elevator to your death. This has been prevented by not letting you jump at all.
 * Throwing a propane tank, a gas can or an oxygen tank at anyone while in the elevator in The Hospital would cause them to fall through the floor, often falling to their death.
 * If you throw a propane tank at a Witch and shoot it, the Witch will disappear, but you can still hear her crying.
 * Switching weapons while using your melee attack would allow you to attack faster than normal.
 * Hitting the Minigun repeatedly with your melee attack and jumping onto the Minigun would launch the player a certain distance, depending on the number of times you hit it. By using this glitch, it was possible to skip decent-sized chunks of many maps, including the forklift crescendo in Death Toll and the door in No Mercy, as well as get to out-of-reach places.
 * "Grenade Spamming" ― If you tossed either a Pipe bomb or a Molotov, then immediately picked up another, you could continuously spawn new grenades. This was very useful against hordes.
 * "Ghost Scratching" ― You're able to claw a Survivor while still selecting a spawn point in the game, and they cannot defend against it at all (Now you spawn near the Survivors, not on them). Fixed on 9/22 update.
 * In Left 4 Dead 2, there is a way to have more than four Survivors. You must first have a defibrillator and dead Survivor. Go to a rescue closet, and the second you open the door to the closet, start to revive the dead Survivor. You will bring back two at the same time. This can be repeated as long as you have defibrillators (now when you save the Survivor in the closet, their corpse disappears).
 * In Left 4 Dead 2, after being in the close vicinity of an explosion, rapidly changing weapons would cause the player to reset stumble animation and move him backwards until the player collided with a wall or if weapon cycling ceased (allowing entry to normally unreachable areas).
 * M60 Replication ― On any survival map of The Passing, there was a way to refill the M60's ammo. If you picked up a weapon close to the spawn point of the M60, 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. This was patched for the PC on the 06/09/10 update, and then on the Xbox 360 with the 07/02/10 update.
 * 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. This can be difficult to accomplish. (This is still possible on the Xbox version of the game.)