Monsters fleeing

The monster AI in Doom causes attacking enemies to reverse direction and feint when the direct route to their target is obstructed. This normally adds to the realism and difficulty of combat (the Cyberdemon on E2M8: Tower of Babel, for instance, could otherwise be killed through a sniper hole), but not always.

A demon attacking the player inside a passage 64 units wide can be turned back by damaging (pushing) it into a wall. This behaviour can be experimented in Doom 1 E3M1: Hell Keep and is a way to escape at the start of E3M3: Pandemonium, most notably under -fast setting or Nightmare! skill level.

Any mobile monster at all, having repeatedly crashed into a wall or other monsters in its attempts to reach the player, eventually turns around and runs away from the player, sometimes reaching the opposite wall of the room before deciding to regroup.

The teleportation trap on MAP24: The Chasm exploits the fleeing behavior as a feature, by putting the room's only exit behind the monsters.

Though not related to chasing AI, but to Doom physics, when a Lost Soul, Cacodemon, or Pain Elemental takes damage, it often drifts backward a considerable distance before regaining its bearings.

Example
[[Media:MonsterFlee.WAD|MonsterFlee.WAD]] (file info) illustrates an extreme instance of this phenomenon. Without it, 100% kills would be almost unattainable.

The Demon in the first room is actually three Demons at the same location, one for each difficulty class. As long as the player does not move too far north or south before awakening the Demon, he will run eastward (away from the player) as soon as he is aroused.