I have a couple.

One is a Great Old One pact Warlock, who finally found that tome of infinite eldrich knowledge. When he read it, it broke a piece of his brain. It didn't kill him and somehow didn't drive him completely insane, but now he has difficulty controlling his mouth and facial expressions. He can't control what he says anymore or when he says it, and is normally spouting an endless stream of eldrich gibberish. He gets around this by communicating telepathically, since he can no longer speak normally.

But sometimes...the gibberish starts making sense, and that part of him can still exert some control over the body...

The other is a heavy metal College of Valor Bard from the far north. Basically think fantasy Nathan Explosion who doesn't quite understand how his music makes people's heads explode, or why he has a bunch of skeleton groupies and roadies following him around. But he also doesn't care, since he dumps Wisdom and Intelligence.