After all these years of game play Nystul discovers a previously un-remarked and certainly un-recorded bug in Diablo -- the rare case of the reappearing foe! Turns out that if a monster (or monsters) is destroyed by a trap (exploding barrels, arrow-trapped coffins, etc.) and you leave the level, when you return that monster "killed" by the trap will be revived by the game! Whoa! To get the story directly from the source, read here.  Amazing pick up on that bug, Nystul!