It's happened a few times on Danny Phantom. In one episode Danny and Sam get into a big fight and she wishes she'd never met him (which turns out to be a big, destiny altering mistake).
In Tiny Toon Adventures, Babs quits the show and joins a cheesy teen sitcom (13-Something, I think) for awhile (I can't remember why, just that there was an argument involved with Buster and she wanted to "show him").