Of course they are! Anyone who invests as much time, effort, money, ego etc. into an endeavor whose result - Smash Hit :) or Turkey :( - is mostly outside your control is going to believe in luck. It's a thing.
The most famous of these superstitions is, of course, not speaking the name of That Play inside a theater. (Or outside of a theater if you can help it.) You know, the McPlay. The Scottish Play. And don't say the name of the principal characters either. Call them "the Thane and his wife" or "the McBs" or something.
This is a superstition I don't particularly share, but some actors get so visibly upset when it's crossed that I go along. When I remember. (I've goofed and had to spin three times and spit and whateverall.) No, I'd be happy to design The Scottish Play again. It's Stephen King's Misery I balk at: I absolutely know that story, set in the aftermath of a car wreck, has nothing whatsoever to do with my totaling my truck driving home from that theater in the rain... but once is enough for me, thanks.
The one little silly theater superstition I do have - one I suspect I share, unadmitted, with a few other set-people - is that all really good shows n' sets get a little blood on 'em. In ancient days a human sacrifice slaked mortar at foundations of great buildings or bodies were buried under stones, as at Stonehenge. This seems barbaric and unnecessary today, of course, because, despite OSHA, nearly all major architectural or engineering projects cost a life or a serious injury anyway, even when trying not to shed blood. Great construction takes its toll.
All My Sons took its blood and was a terrific show. I stapled myself to the wisteria-draped set of Enchanted April and that was great. A dancer sprained something for the wonderful Urinetown. Now Lovers & Executioners, which has already claimed a broken toe (kicking the pile of research books counts as play-related, no?) has now collected a bit of blood from me too...
By golly! I demand apparent "broken bottles" on the top of that garden wall for this play about adultery, but I thought the plastic bottles I was breaking would be safer.
Vintage good luck card from Freestock.ca
The other famous theater superstition? Never say "Good Luck"! Say "Break a Leg!"
Or, in this case, a Toe.