Cliverjo - // You mustn't whistle in a theatre, either. Very bad luck! //
Indeed, theatre folk are notoriously superstitious.
If you accidentally whistle in your dressing room, you have to go out and close the door, turn around three times, knock on the door, and enter.
The most famous superstition is not mentioning the play Macbeth by name - only as 'The Scottish Play'.
It was believed that its mention would shorten a production run, because if a run ended early, a regional theatre would often put a Macbeth production on as a safe bet to get the punters in until the next show started.