[SixOfSpades]

You might also run into problems if you hack the avatar of a creature that does not normally cast spells (such as a Cow) onto a spellcasting character. When you start casting, the game will look for the animation frames of "Cow casting spell," not find any, and freeze up.

A similar problem occurs in avatars that have no "death" animation, although these are much rarer: Elminster is the only avatar I've heard of that will crash the game upon death.