Just in case OP is actually serious and not a troll.... In real life, not very often. You probably will have it happen once in a while by accident. I was doing BJJ the other day and there was a guy who accidentally kneed his partner during a takedown. In striking styles, somebody might throw a kick wrong sometimes and hit you there. But real life isn't like Master Ken's show - people don't actually practice "stomp the groin" techniques in class with a partner.