At a game last season, I called a batter out on the infield fly rule. After the inning was over, a parent walked over to talk to me behind the backstop, they were very upset that their kid was called out when the infielder never made a play on him. I briefly explained the IF fly rule, and the lady was still upset. (I just walked away and let her talk to the coach)
In your opinion, what is the most unknown rule in baseball?
(I don't think the IF fly rule is the most unknown, it's just an example)
thanks