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.

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.This also applies for survivors.
 * 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.
 * On occasion if you touch an object previously punched by a Tank, you may be killed.
 * "Crouch Running" - It is possible to crouch and go running speed as both Infected or Survivor, this glitch was not removed with an update because it is virtually un-exploitable.
 * A strange turret glitch with the new campaign Crash Course, as seen in this video. 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 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 game (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 2 or more people are playing and 1 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 A.I. do not open Safe Room doors, most likely to avoid irritating players.
 * 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.

Left 4 Dead 2

 * 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.
 * If you deploy an ammo pack before you end a Chapter in a safe room or anywhere you can re-visit you can actually get two lots out of it as long as it is still there, so 3 people can take from it, but it must still be there. This can be useful on finale's with the safe room close to the actual Crescendo Event in case you need some extra fire power. (Works with Incendiary Ammo, Explosive is untested.)
 * 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.
 * 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 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, the survivor can get on top of the elevator.This may also happen while being revived.
 * It is possible for 4-5 of the same Special Infected to spawn at the same time in Campaign mode. This is currently unconfirmed for the Tank and Witch.
 * Rarely in The Concert, the final horde and 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.
 * 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.
 * Sometimes on Dead Center: The Hotels, the Survivors will still use their friendly-fire sounds from before they introduced each other. Ex: Rochelle will say "Hey, what's your name, don't shoot me!" despite knowing everyone else's name.
 * 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...

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 clip 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 molotovs and can be very useful in a Crescendo Event or a finale. Fixed on 9/22 update.-Appears to be usable on Xbox360 &.
 * 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.
 * 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.