'Break Your Leg'

I recently taught idioms to my Level 10 English class such as; break a leg, knock on wood, a bed of roses, bury your head in the sand, a back seat driver, to play it by ear, to see eye to eye, etc.

Later one of them practising their English emailed me a short message that included the ending - break your leg!

It is always fascinating how 2nd language English usage can get so entertainingly twisted.