One of the many things that is considered bad luck in a theater is whistling, supposedly because many early stagehands were sailors who used whistled signals to communicate. Does anyone know of an original source that might confirm this? And if it is true, are there any examples of what it sounded like?
There are a broad spectrum of occupations and circumstances in which whistling is regarded as bad luck or ill advised. The wiki article includes a few with fairly good sources. Hamilton Jenkins also notes that Cornish miners forbid whistling in mines. And whistling in the dark in many Western European cultures (and parts of North American) is regarded as bad luck. Informants often indicate that whistling will attract supernatural beings, and that attention can be dangerous, which is why one shouldn't do it. I suspect the occupational prohibition associated with the theater - which is an unusually powerful magnet when it comes to folklore - is tied to the general tradition, which is much larger than the maritime industry.
While stage rigging before 1882, and the revolution begun by J Clancy, was largely based (and likely operated by) sailors. The idea that sailors communicated by whistling is a misnomer. In fact, according to the British Navy, whistling on board was a punishable offence (until 1910).
So at least in Britain, there is no solid basis for the superstition.