It's hardly unusual. Back in the 1980s a friend of mine was responsible for arranging an Under-11s match between his own school side and that from another school. He suspected that things might get a little bit 'lively', so he asked me (an ESFA referee) to take charge of the match.
The very first tackle of the game was completely fair but it left an opponent sprawled out on the ground. (It was a slide tackle where the ball was played but the opponent simply fell over the legs of the boy making the tackle). Unfortunately it happened directly in front of the father of the boy who had been floored, who immediately went berserk about what he saw as an attack upon his son. Even more unfortunately, the father of the boy who had made the tackle happened to be standing right next to him and they were quickly trading blows. That was only 30 seconds into the game!