Did the live magnetos thing. On when they should have been off.Flat battery. Man swinging prop for me, just sucking in, of course it started up. Fortunately he was treating me like an idiot, and was standing back when he released the prop. Came round the door, gave me a polite telling off.I flew, landed, and sought him out. He was in the middle of a large group of people on the ramp [he was someone quite famous in the flying world].I swallowed my pride [what was left] walked into the middle of the group, and apologised to him. As I walked away, several people asked him what that was all about. I heard his reply 'oh, nothing. He just learnt something that he won't do again'.He could have made me appear a fool to so many people, but chose not to. Nicest person I ever met, flying wise. Happy New Year.