Break a Leg!

So far in my life I have broken my foot, my wrist, and my rib (at different times) and all have been unpleasant experiences. 

There is one area where wishing for someone to break a bone (a leg, specifically) is good: in the theater. 

The expression "break a leg" is positive encouragement, said to actors and actresses for good luck before they go on stage, especially in live theater productions.

This entertainment idiom has various unverified origins, however.  One theory is that performers in show business tend to be superstitious, so saying, “good luck” is believed to bring bad luck, By opting for a cheery “break a leg” instead, it is expected that this negative expression will bring good luck.