This isn't strictly speaking a bug, but is something sufficiently exploitable to warrant consideration:

Virtually no monsters have resistance to confusion, blindness, stunning, poison, or paralysis. These include all 5 orb guardians and Andor Drakon. Considering how easy some of these effects are to produce (particularly confusion and blindness can be achieved from common potions), this makes for a very easy way to dispatch of many of the game's most dangerous opponents.