It really annoys me when people run their mouths at sporting events and in doing so show that they have no idea what they are talking about. I have been known to turn around and ask such an idiot "you don't know much about the sport of ____, do you?".
One such moment came in Toronto a couple years ago at a Yankees/Blue Jays game. The guy behind me kept screaming loudly that Andy Pettitte kept balking. I turned around and said something along the lines of, "Dude, shut up. There is no one on base. Try reading a rule book before you start yelling things and making yourself look like an idiot."