Here is a topic for life lessons you have learned the hard way.
In a fight, even untrained persons can get lucky, either getting a fluke hit on you, getting you when you're not at peak condition, or both.
(Self defense class. There was this girl who joined because she received a gratuitous beat down from a girl gang. I was assigned to train with her. So she had to learn how to hit an attacker to make him let go of a physical hold he has on you. The purpose was to learn the move, not actually hurt the person. I hadn't slept well last night, so I didn't get my cue and let her go on time. She panicked and hit me for real. Was knocked out cold for 10 seconds. 1à seconds is a pretty long time in a fight. Had this been a real fight, she could have done whatever she wanted with me during 10 seconds, despite the fact that I had the training and physical fitness advantage over her. She apologized when I came back to the world of the living, I had a laugh about it, saying that accidents happen and if I didn't wanted to get bruises I wouldn't go to a self-defense class, and I was pretty tough. Still, I had to explain a nasty bruise on my eye to all the people I know. I had no reason to lie, and I laughed and still laugh about it. It was actually a good memory. Still...)