Whose Line is it, Anyway? (US) - Ow, That's Gonna Leave a Mark
In one episode, during a game of party quirks, Ryan is lying down right in front of Drew's dsk. He gets up a little bit, and hits his head on the neon thing on the front of the desk. He breaks the light, but stays in character. Go Ryan! After the game, Colin says," It'd be better if your head just exploded."
Actually, Ryan was supposed to be playing a Carol Channing whose head keeps getting stuck to things. He was trying to make it look like his head was stuck to Drew's desk, and hit the light instead.
This isn't really much of a slip-up. It is pretty much Ryan being Ryan and funny and stupid! Notice everyone was cracking up for the rest of the show! The point of the show is for people to crack up. So It wasn't really a slip-up, it was a really funny thing!
(once again your hardly going to find a slip-up on an improv show)
Nick