This may not be up to 'pro' practical joke standards but it sure had me buffaloed for quite a while. For many years myself and some friends would play music on weekends at each others houses. There was always enough non-players to make up an appreciative audience. I had SO much trouble staying in tune a lot of the time, that it almost drove me crazy, and when my buddy suggested better tuning pegs, an electronic tuner and even a new guitar, I'd listen. One night I found out WHY I couldn't seem to stay in tune. Every time I'd turn my head around my buddy would reach over and loosen one of my tuning pegs! He must have gotten really good at it because no one in our little audience ever noticed. (I think) They WOULD have told me what he was doing, wouldn't they?

marty