Way back in high school I was riding my BMX down a nice steep hill, (we used to chase and pass cars on it) and I ran into my football coach who was coming up the hill in his little Honda Civic. I remember nothing from about 5 minutes before the wreck till I woke up the next morning with a cast on my arm.

Back in grammar school I went head first into a fence post holding up the chain link fence playing touch football and lost the whole school day, my memory started working again at some point during football practice after school.

I haven't had any concussions with long periods of memory loss as an adult although I've had a couple where I lost a few minutes.