In my teens there was a guy who kept talking shit about me (and had already provoked me into fights in the past) and then thought it was funny to grab me around the waist from the front and lift me up. I lost it and repeatedly punched him on the top of his head as hard as I could until he let go, while my sister was shouting at me to stop it.

He later brought it up at a time when he wasn't being a dick, in a light-hearted "remember when you..." way, and I felt like I should have apologised then, because it was an over-the-top reaction and I potentially could have done some serious harm, but in my social awkwardness I didn't manage to say it. I think he respected me after the incident though, and never messed with me again.