Doom - Sleeping Sergeant

The Sleeping Sergeant glitch is a simple error in Doom II's programming.

Description
In MAP02 of Doom II, after going through the first door, the player must get past a sergeant in order to press the switch that opens the next part of the level. However, this sergeant ignores the player and does not "wake up" as enemies normally do when spotting a player.

Why it works
Usually, the bug is unnoticeable, as the player must confront two troopers in the same room before reaching the sergeant. The action of killing them usually wakes up the sergeant (who is still sensitive to sound).

The same error can occur, in reverse, on E1M5 of Doom I. If the player walks slowly up the left side of the first staircase, he may awaken the Imp in the far northeast corner of the room directly in front of him, even though there is a solid wall between the two locations.

Credit

 * The moderator at Doom Wiki for letting us use this info.