I sort of hit my Biology professor today. We (the other students, I, and he) were in the lab. He touched my shoulder - I flinched and said "Don't touch me" - I think he thought I was joking, so he touched me again. I elbowed him in the stomach.
He apologized, and things went on as usual. I actually get along with him really well. He wasn't inappropriate - I just don't like to BE touched, although I don't always mind initiating touch. Chances are I once touched him and he therefore assumed it was OK he touch me.