Superstition

Uh... no it has nothing to do with SMs. It has to do with the fact that the riggers and flyers who operated the flies and ropes would use a system of whistles to cue each other and let each other know what to fly out and what to fly in. If you whistle the wrong thing, it might cue one of them to accidentally drop a sandbag, set piece, backdrop, or other such item onto someone's head. (I thought this was common knowledge... apparently not :)) Puppets and patterns at Puppets in Melbourne