I've had a couple of nasties.
Several years ago I was on the last leg of the Marin trail when i came to those BMX type humps on a downhill section. I was trying to wheelie over them but as the back wheel ran into the hump it kept kicking up and making me land on the front wheel. 'Hmm, I seem to be getting away with this' I thought, 'I'll go a bit faster' BIG mistake. Landed over vertical on the front wheel - what can you do then? I'll tell you. Massive face plant at the high end of 20 odd mile per hour, threw me into the side of the bank and spun me and the bike into a metal sign post. I lay there dazed for a few minutes rubbing my thigh until my mates caught up. When I tried to get up I couldn't! My leg just wouldn't support my weight. My mates helped me onto my bike (the rails of my saddle were leaning at a crazy angle) and I rolled down the last couple of miles of switchback STILL beating them to the bottom. I couldn't pedal though, couldn't hardly walk. A mate drove me home the next day my quad was like a huge inflated balloon, turns out it was ruptured and I also had damaged knee ligaments. Took months of painful physio to heal.
My other nasty one was going too fast in Sherwood Forest of all places. A similar incident (you think I'd learn) I wheelied over a log and the back wheel rebounded off it and chucked me over the front. I KNOW I should have bunny hopped it! I hit the ground and the bike hit me in the temple - knocked me out! When my mates caught up I was spark out but then apparently started groaning. I eventually got up and dusted myself down to find that my shoulder didn't work properly. Also it became apparent to my mates that something else was amiss. I kept repeating myself, the same thing over and over again. Back at the car my mate rang my home really worried now. My Mrs is a nurse in A&E and gave him some info. Turns out in was concussion and short term memory loss. Spent the evening in A&E, they only let me home because of the Mrs.
My friend Bob had given me a saddle to try out and it was in my other mates car who was driving me home from Sherwood. All the way back I was saying to him 'Nice saddle, whose is it?' Answer: 'It's yours, Bob gave it to ya.' 5 mins later, 'Nice saddle.... etc etc etc ad infinitum.