"BREAK A LEG"

Not literally, Uncle Woody.  

It is considered bad luck to wish someone good luck in a theater.

In English-speaking countries, the expression "break a leg" replaces the phrase "good luck", which is considered unlucky. The expression is sometimes used outside the theatre, as superstitions and customs travel through other professions and then into common use. If someone says "good luck", they must go out of the theatre, turn around 3 times, spit, curse, then knock on the door and ask to be readmitted to the theatre.

The exact origin of this expression is unknown. One theory is that "Break A Leg" is a very old military term for "taking a knee", or bending down to one knee and breaking the line of the leg. In the theatre it is a reference to "taking a bow". To wish someone to "Break A Leg" is to ask them to give the best performance they are capable so that they may deserve to take a bow at performance end -- or, to "Break A Leg".

This expression has so entered the mainstream that it is used by non-actors toward actors and in non-theatrical situations, however, to say "break a leg" in ballet is considered just as much bad luck as saying "good luck", considering it's a physical art.