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.
 * On the No Mercy Campaign, on the Hospital chapter, while you wait for the elevator, if you can find a Oxygen tank in the hospital rooms. While you are taking the elevator up, it is possible to throw the oxygen tank at someone that instantly incapatiates the person hit by the oxygen tank making them fall through the floor and die.
 * 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 usefull at this.