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.
 * 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.
 * 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 (maybe the 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 are on the Infected team, and the single Survivor must not be the host, now the Survivor goes to the menu to change characters, then has the host start it and he changes to Infected. This is actually still possible and it is also 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), apparently the AI are too stupid to open the safe room doors.
 * When you heal someone, and he or she gets constricted by a Smoker, you are still healing them although they're dragged away by the Smoker.

Left 4 Dead 2

 * In the Left 4 Dead 2 Demo, there is a way to have 5 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.

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.
 * 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.

Bug Usage
Please keep in mind that in Versus, using bugs that give you an advantage is entirely unfair and is looked down on by most players.

Try to limit using game-changing bugs to single-player.