Several Doom WADs exploit a bug to create monsters that are all but unkillable. This spoiler describes the bug and how to deal with the nasty critters that result.

Arch-viles have the nasty habit of resurrecting dead monsters. If a monster was killed by being crushed, or was killed normally and then crushed, it has some particularly ugly properties when it is revived. It can walk through most walls, and weapons fire goes right through it. So you may find these critters inconvenient to deal with.

One way to kill them is to fire a rocket at an object close to them. The backwash from the blast is capable of doing them in. Of course, it's not real healthy for you either, so if these fellers close in on you, you're in deep yogurt. They have also been known to be killed by other monsters.

Two WADs that exploit this bug are Waterfront, and level 24 of Icarus. Thanks to Scott Crank, author of Waterfront, for the explanation of the bug. You'll find more details in a text file accompanying Waterfront.