Yeah, a couple, a long time ago.
First one when I was about 12-13 year old. One kid, who I'm good friends with now, were making fun of me when I was out cycling for wearing a helmet. I headbutted him in the face with my helmet on, tripped him and punched his nose twice. He bled badly, and I felt really bad about afterwards when I realized what I've done.
Second fight was with the same guy actually, some years later. We and some other friends were playing football (soccer), and that guy launched a two-foot tackle on me from behind. I told him that stuff like that isn't acceptable when we're just playing for fun with friends, and if he did it again, I'd make him stop.
He did it again. And I punched him in the face. He went home after that.
Sidenote: We're good friends now, honest. And I'm not a violent person, double honest!