Trapped monster in PC TNT map 29, River Styx:

On the west half of the map are two narrow hallways, which
are physically separated from the rest of the level and are
normally inaccessible to the player.

Both hallways contain monsters and teleporters.  A player's
weapons can be heard by these monsters once the player gets
close to the level's exit.  In general, when the monsters
hear the player they'll begin moving, eventually step on
the nearby teleporters, and reappear near the exit chamber.

An exception occurs at Skill 3 ("Hurt me plenty"), where a
Mancubus monster is too wide for the hallway, and is unable
to get to the teleporter.  Because of this, a maximum Kills
score of 98% is normal for this map at Skill 3.