Certainly one of the funniest guitar mistakes was made by a friend of mine who learned, to his cost, the folly of trying to play jimi-hendrix-style with the guitar behind his head.
As he attempted to whip the guitar over his head, mid-solo, he realised, all too late, that his guitar strap wasn't long enough and hit himself smartly across the forehead, leaving a very nice stratocaster-horn-shaped bruise!
The oddest thing about the whole thing was that he admitted doing this to me and his mates in the pub!!
Suffice to say, we won't ever let him forget it!