Just once, and it was before I began studying martial arts. I was in my early 20's, and a bouncer in a somewhat rowdy bar. I was actually by far the nicest bouncer in the place, and had a reputation as a peacemaker.....until a jerk came in one night and smacked one of our waitresses and knocked her over a table. It didn't help matters that she was also my roommate's girlfriend. I went after him like a rocket. I don't remember what I did to him (this was almost 30 years ago), but he was in the hospital for 3 days. The chief resident called the bar owner and asked what the hell we did to the guy.
Looking back, I'm not sure if it would have been better or worse if I had been a MA student at the time. On one hand, I might have had more restraint, but on the other hand, I also might have killed the guy right on the spot.
I'll do just about anything to avoid a fight, but if I see you strike a woman, all bets are off.
I'll also say that was the only time I've ever hurt someone and felt absolutely no remorse.