The biggest opportunity for fights has got to be whenver guys can't slow down after the finish and they blow by you in the chute (even though they didn't actually cross the line ahead of you). This is annoying as hell in non-chip races, and you know the guy is going to get placed ahead of you in results.
With adrenaline pumping, it always takes significant self-control to keep from decking these guys. I got to believe that happens all the time in HS/College XC, never been there though.