I had a few in HS. One sent me to the hospital as I was playing basketball and hit a brick wall behind a woefully inadequate gym mat. I woke up with my dad, my coach and one of my friends looking very concerned. Don't remember much beyond feeling very dizzy and woozy for a few days.
The one that scares me the most, I was returning a punt in practice and got smacked good. I didnt come out of practice, drove home and went to bed and I have no recollection of that. Lot of things could have gone wrong there