We've all seen the classic bug where a mob goes into a blind rage, attacks itself, kills itself, then somehow looks more powerful. Here's a strange twist I've never seen before:

Code:
The cave lion cringes in pain! The cave lion decides to fight to the death in
a blind rage! The cave lion glares at the cave lion in a very strange
manner.
The cave lion was one I tamed.

Copied and pasted this in before I did anything else, but apparently it get more interesting. This seems to trigger a bug that causes a game crash every time the affected monster has a turn. I can reload the saved game, but it keeps crashing.

Crashes went away when I killed the cave lion with a thrown rock. (Crap - so much for the cat ring - the whole reason I tamed the cave lion in the first place.)