On the other hand, the brain is a tricky thing. A buddy of mine crashed badly on the track. I sat next to him while we were waiting for the ambulance, and we had about 10-minute conversation chatting about various subjects. He was completely lucid and sharp, cracking jokes about getting a new bike, etc.

The next day I visited him and he had zero memory of our conversation.

I've learned to never, ever trust what someone says after they hit their head. Always assume they're bleeding/swelling internally. Damage is not limited to the initial trauma, but can take place over hours or even days after.