I once had a very serious fight with my brother; we'd actually punch and kick and stuff, and then my mother got between us and stopped it, and my brother and I didn't talk with eachother for a pretty long time. It was that serious. If he hadn't known any Tae Kwon Do, the fight would have ended sooner with me as the victor (you see, I Karate chopped his sorry butt). Unpleasant, but funny times. I still remember what triggered it, but it's a long and complex story and I'd rather forget. So yeah, other than that it has never been as serious, I think. Fights are bad, someone always gets hurt :/. So I'd only ever resort to fighting if I absolutely had to.