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 4-5 glitches floating around right now. I will not explain how to execute them.

Power Shot- You are able to fire all 50 shots in your clip simultaneously with an Assault Rifle or Uzi with extreme accuracy. Assault Rifle deals 1500-1600 damage, while Uzi 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.

Teleswiping- You're able to swipe a survivor while still selecting a spawn point in the game, and they cannot defend against it at all. Also known as the "WTF is happening to my health!?" glitch.

Boomer Dump- Boomer Bile somehow hovers around your character and constantly biles you. As soon as it wears off you'll be biled 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.

Bugs that need confirmation
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.
 * On the No Mercy Campaign, there is a room near the helipad where the door cannot be opened but it opens itself. When in this room the player(s) will not come to harm. This room is known to many as the "Jesus" room.
 * 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.
 * If you throw a propane tank at a witch and shoot it, the witch will disappear but you can still hear her crying.

Fixed bugs
The following list of bugs are the more notorious ones that have been fixed as of the most recent patch. For a comprehensive list of things that have been fixed or added to the game, see: Update info.
 * 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.
 * 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 rose but they did not. They would also have been unable to leave the elevator shaft, and have been easy prey to any other Infected.
 * Switching weapons while using your melee attack would allow you to attack faster than normal.
 * Louis's body sometimes gets stuck in the safe room door.