Whistling backstage in a theatre is considered a jinx.
This superstition grows out of a practicality: In the good old days of theater, scenery was manually lifted into the air by men hoisting it with ropes (there were no hydraulics or advanced rigging systems to make it safer and easier). The stagehands would cue each other by whistling. An actor who whistled backstage might accidentally cue a stagehand to lift or drop scenery, potentially putting an unaware performer at risk of being crushed by a wall or a sandbag. The best way to make sure you didn’t become a theatre ghost was to refrain from whistling altogether. The rule has stuck and become a superstition.