“You can never say ‘good luck,’ you have to say ‘break a leg.’”
My informant grew up in the theater. This is perhaps the most common superstition among stage performers. Although my informant wasn’t sure exactly where the tradition comes from, it is likely that performers don’t want to jinx their show by talking about luck or anticipating a good show. Instead, “break a leg” does just the opposite – it wishes harm on the performer so that the opposite will happen.