A few months ago I linked to a few opera blooper videos on YouTube. People have recently been asking about them so here is one of the videos once again:
Note: I don't really think these bloopers are footage from real performances--you'll notice the same actors keep on coming back. Alas, they're probably staged for television but are every bit as funny.
Another great audio-only blooper: the legendary Messiah Organist on Crack.