As an addition to the above, the reason why whistling in a theatre is considered bad luck:

Back in the days before cans sets (integrated headphones and a microphone to talk to other people working backstage) were around, the people doing the flying would communicate with each other with whistles. This would give them their cues to lower or raise scenery, or do anything else with regards to flying.

If you were walking across the stage, and happened to be whistling, then one of the flyers might think it was someone giving them a cue, and you may very well get something dropped on your head. Of course, these days, this is not an issue, but, like so many things, it has turned into a superstition.

If you're after any other theatre tech information, then do take a look at 'Everything you ever wanted to know about theatre tech, but were afraid to ask'