Whistling:
Whistling on stage is considered bad luck because in the old days, original stage crews were hired from ships that were in port (theatrical rigging has is origins in sailing rigging) and coded whistles were used to communicate scene changes etc. Actors who whistled could confuse the stage crew into changing the set or scenery. This could result in a set piece or sandbag being dropped on the actor with distinctly unfortunate results. Although today cues are given via intercom or other signaling methods, it is still considered unlucky to whistle backstage, or clap for that matter.
One way to void the curse of whistling is to run around the theatre three times and knock on the door to be let back in. (seems to be the cure for most theatre curses)
So, if you are ever on or back stage remember this tale to avoid having to run around the theatre in the dead of winter.