Monsters on rampages and Witches literally exploding are the two I've heard most often.

[ He may as well get the big stuff out of the way. ]

Those are the extremes, however. Though you'll hear about them most, it seems the most common ones are irritability, aggression, paranoia, failure to control magic, monstrous instincts exerting too much influence...

Truthfully, it sounds quite similar to someone suffering from an excess of stress in their daily life, albeit it with more disastrous side-effects.