Fred was a member of our ceilidh band. He played the guitar with enthusiasm, but not a lot of skill. But he was a great guy. so we never plugged him in and everyone was happy. One day we were playing a newish number, so we warned him that we were going to play it and to listen for it. We played the next number, and said OK Fred, this is the new one. Oh said Fred, I've just played that.

And no-one noticed