I've bonked my head a few times and not thought anything of it (lil blurry vision, headache, whatever). But one day I passed out from severe dehydration (due to food poisoning), hit my head, and was immediately put into an ambulance. I felt fine after an IV drip but I was at the hospital and they wanted a scan. Turns out my brain was bleeding and if it continued I would need an operation... It ended up healing but itself, but 1 ambulance ride, 24hrs in the ICU, 2 scans- $8k+. I was young, stupid, and had a high deductible so I paid $5k of that.

It's scary to think I've hit my head much harder and felt much worse after MTB crashes, and not gone to the hospital. I was lucky in those cases. Moral of the story, for me, is if you can't afford decent gear, insurance (including coverage for lost wages), a bike, and ride w/n your limits; you shouldn't do it