I think most of us have had our fair share of gigs from hell - we once got asked to turn down the music a bit in a club because some of the oldies couldn't hear their poker machines.
However just when!!! while playing in a pub i thought right nobody's listening so said to the other guys i'm going to mumble the whole song - well a guy comes up to the sound tech and says can you do something to the sound we can't understand the words?

You never know!