TIFU by giving my friend a concussion

Obligatory, not a fuck up today.
This happened about a month ago.
I was walking by my friend and tapped him on the head as he wears a cowboy hat and I’m always tapping the hat. I was talking to him as this happened. It wasn’t until the next day that I realized something was wrong when he didn’t sit next to me. I asked him what was wrong and he explained he got a concussion from it. I felt so bad about the entire situation. Because this was at school, I did get called down and they reviewed the video and said that while it was just a tap and it was a very slim chance that it could have given a concussion, the person had a condition that magnified the effect of pressure to the brain. Thankfully, nothing more came of it and I was just asked to stay away for this person.

TL;DR: Tapped a friend on the head and ended up giving him a concussion.