Superstition

Theater people are nearly as superstitious as sailors. There's a reason for most of it, though. Never whistle on-stage - or backstage, come to that. (If the rubes ask why some thespians will tell them that it might call something... or somebody. It's true enough, in a way. Scene-shifters use a whistle code to signal when to strike a scene, lower a backdrop, loose a trap-door or the like. Use the wrong couple of notes and you could be under a counterweight trying to work out what hit you!)

Never use the real title of the Scottish Play, or use the last line of it until the opening night. Never use real jewelry on-stage - there are good reasons for that, and not just the theft thing.
There are dimensions where they never let the star sing a new song or recite a new verse unless an understudy's done it first, in the full costume that's to be used on the night, with all the props in place. (That started after Operative 6 got stuck with taking Dame Vellic Delba to be his bride at the dress rehearsal of The Gypsy Baroness. She weighed about 300 pounds and only qualified on a technicality.)

There are several dimensions where they won't have live flowers backstage - all places where they grow something like ragwort or goldenrod that a whole bunch of people are allergic to. There's no bitchiness like show bitchiness and smuggling something like that into the star's bouquet has been known to set off an asthma attack during the encores.

(Show business ain't always pretty backstage and the thing about not using new makeup when you're going on for real comes from the same cause. Get used to the look of your slap before you have to use it while your mind's on something else. Then if your understudy gets clever with broken glass you've got a chance of spotting it before she's standing in for you after your accident...)

Then there's the time that the punk-rock group used real, authentic witchcraft symbols on the backdrop and called up Bromobrathi. All three of them were singing soprano after that - but you wouldn't want to know the details. It was hushed up anyway, but now if anyone wants to use that kind of stuff it has to be detoxified and blessed before most managements'll let it into the place.

Like I was saying, there's a reason for a lot of the show business superstitions.