I've seen on YouTube where Muse cock up the start of a song. Chris starts playing one song and Dom plays another, both songs have bass/drum intros so unless you're looking for it, it probably wouldn't have seemed wrong. Funny though.
Another is Green Day's Mike Dirnt playing longview on the bullet in a Bible gig, he is moving up/down the steps and plays a duff note, sounded horrible but kept going. Found that funny too, it's nice to see the pros cocking up too!