I'm sure that this has been discussed elsewhere, but I thought I'd include it for completeness:

Monsters who go berserk and enter a blind rage will occasionally target themselves rather than the PC. This often leads to funny messages (eg. The doppleganger looks like the doppleganger; the doppleganger is confused; the doppleganger attacks the doppleganger; the doppleganger is killed), and causes little harm except to deny the PC the experience resulting from killing the monster themselves. If it happens to a quest monster (eg. Kranach), then the PC doesn't receive the reward.

More seriously, if this bug is triggered by a companion, a near-fatal crash is triggered. The game will automatically save and close, and will do so every time the player tries to restore the game.