I work in theatre. There are a lot of superstitions surrounding the stage, some of them so old nobody knows where they started. I, personally, am not superstitious, but I love learning about the origins of them.
It is bad luck to put shoes anywhere but on the floor--it curses the actor wearing the shoes. It represents the feet being higher than the body, which equals falling onstage.
It is bad luck to whistle backstage. This one has its origins in the early days of "fly houses," when they first started using ropes and pulleys to "fly" the scenery in and out. Sailors were often hired to man the ropes and they communicated scenery moves with a series of whistles, just like they did on ships. An unintentional whistle backstage could result in chaos. Whistle cues went the way of limelight and corsets, but the superstition stuck, and now if you're caught whistling backstage someone may ask (or demand) that you spin around three times and spit behind you.
A theatre is never, ever left completely dark. After hours, a "ghost light" is always left onstage. Depending on where you are, the ghost light is to either ward off ghosts, or to invite the ghosts of former stars to come back and relive their days onstage. There are also those who take the word "dark" literally. In theatre-speak, when a theatre is "dark" it means there is no show running at the moment and there's nothing onstage--meaning the theatre is not making any money. Therefore, some people never allow the theatre to go completely dark. If there isn't some kind of light on at all times, the theatre will shut down. (These days, it's more of a safety thing--theatres are cluttered with scenery, props, and all sorts of dangerous objects, and it's just safer to always have a light on.)
And everyone knows the phrase "break a leg." It's not really a "bad luck to wish good luck" thing. Back in the days of vaudeville, the performers only got paid if they were seen onstage. The long, narrow curtains on the sides of the stage are called "legs." Therefore, if you "broke a leg," or made it onto the stage, you got paid for the evening.