Bugs

Glitches are an unwanted result of coding or programming of a game. There are many glitches in Left 4 Dead that can be used for achievements, an advantage in versus, or just for fun. Glitches can be preformed on any system without the use of a command prompt on the PC. Glitches are normally patched out by a download that fixes them, so normally very well known glitches won't exist for long.

Common glitches

 * It can be observed that common infected can sometimes just freeze in the position they were in. They continue to growl, but just freeze in that frame of time.
 * 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
 * The turret glitch is when the player constantly hits a turret to "charge" it up. when the player has done this enough times jumping on the barrel should send them flying across the map or up into the air. This glitch is useful on the Dead Air Finale because you can jump up onto the aeroplane from harm's way