I did by crashing into another person on a bicycle when I was riding and hit the back of my head on the road. I felt a bit nauseous and spent about 10 minutes

I did by crashing into another person on a bicycle when I was riding and hit the back of my head on the road. I felt a bit nauseous and spent about 10 minutes lying down in someone's back yard. After that, I walked home. That was close to 40 years ago and I'm still fine. For what it's worth, I still ride about 300 to 400 miles per month without a helmet.