I climbed on top of shed when i was younger and since it wasnt sturdy and quite rickety (after years of me and my friend spending long afternoons on top of it) anyway i fell off and landed directly on my back on pieces of metal, wood and car engine (his garden always had junk in it as his dad was a car enthusiast)...i was paralyzed for 10 minutes and had a bad back for months after.

Ive had loads of other painful experiences like that but that was the most scary/painful and with the longest lasting effects.