Sophomore year, I was a flanker on the JV team. Last week of the season, and I was running scout O against the varsity D. I took a handoff and ran through the line (I had never taken a handoff before) and got absolutely creamed helmet to helmet by a senior LB twice my size.
I was knocked out and fumbled. I came to and the guy who tackled me offered me a hand and pulled me up to my feet. One of the assistant coaches noticed I was still woozy and told me to sit out the rest of practice. When I went into the locker room afterwards, I couldn't remember the combination to the lock on my locker.
Over the next few weeks I had headaches and I didn't feel right. But this was 1985, and we didn't go to the doctor for such things. Eventually I got back to normal, but the next year when it was time to sign up for football, I decided to switch to cross country instead. That hit stuck with me and I wanted nothing more to do with it.