My personal favourite:
"You are studying medicine. Are you going to be a nurse?"
Why don't they just say:
"You have ovaries/breasts/two X chromosomes. You MUST be a nurse!"
Clearly these are questions asked by people who have not met the awesomely kick-arse male nurses who roam the wards.
Actually, I'm sure they have met them. They just mistook them for doctors. (Or bouncers.)