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.

Current bugs
There are around 7 glitches floating around right now. This will not explain how to execute them.

All versions

 * "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. Also known as the "What is happening to my health!?" glitch.
 * "Boomer Dump" - Boomer Bile somehow hovers around your character and constantly vomiting on you. As soon as it wears off you'll be vomited again.
 * "Hunter Dropping" - You can hide on a survivors head and pounce them for 25 damage, usually only achieved by dropping from an insanely tall building.
 * "Witch Surfing" - You can ride a witch while she's angry and basically be a Mongolian riding a war horse. Very devastating to inexperienced players.
 * "Speed Crouch" - You are able to move at high-speed while crouched. This gives the Hunter the added advantage of moving silently.
 * "The Jesus Room" - There is a room in No Mercy on the Mercy Hospital rooftop that allows all four survivors to "wait-out" the finale.
 * 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 touch the vehicle, you die instantly for no apparent reason.

Xbox 360 version only

 * "Power Shot" - You are able to fire all 50 shots in your clip simultaneously with an Assault Rifle or a Submachine Gun with extreme accuracy. The assault rifle deals 1500-1600 damage, while the submachine gun does around 1000. Be wary when using a Tank on Versus mode, as this has become very popular and can devastate even the most experienced Tank.
 * 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.

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.
 * "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.
 * 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.

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.