If you are backstage and want to fill the cast and crew with extreme dread, all you have to do is shout, "Good luck!"
Some thespians are very superstitious. They believe it is bad luck to say "good luck" to a performer, especially on opening night.
If you want to wish a performer well, say "Break a leg" instead.
Sounds violent, doesn't it? Yet, it's a traditional term of encouragement. There are many theories as to where this saying originated. Wikipedia's article explores many possibilities.
So, if one says "good luck" by accident or by choice there is only one antidote. The perpetrator must go outside, spin around three times, and spit over his left shoulder. Then, and only then, will the cast allow him back into the theater.