My first real crash was on a triple butted Tange steel framed MTB with a Rockshox Mag 21 fork (I built it myself) back in november 1998. A guy on a fully suspended Votec downhill bike led us over a downhill jump. I had ridden the jump a year before, but our guide had "improved" the jump after that, giving it a steep lip and doubling the height. I was first to go after him, but came to fast: flew 10 meters downhill, doing a sommersault and landed head down. The bike continued for another 10 meter. Lost all air afterwards for half a minute (roaring from pain). After getting back on the bike, and riding over roots, I could feel the indirect pain in my spine when I hit bumps: So i knew my back was broken. We rode back to the car, and called an ambulance. Went into shock at the ER due to pain, but morphine helped.
My back has become weaker, I get back pains almost every day but then again **** happens, and biking is fun.
Mountain biking can be dangerous. Racing uphill is not dangerous (if you snap anything, it will be the chain), the real disasters always happen going DH. In my old MTB club during the 1990es, we saw one permanently shattered pelvis (girls doing DH in Norway), one tetraplegia (16 years old boyracer talent crashing sideways DH into huge tree. Is now in a wheel chair (broken neck), one punctured lung (adult, alone in the woods, huge tree, DH, found by XC runner before nightfall. of course, we were more than a hundred members, but the "toll" was quite high. At the time, we all had good helmets, but no other protective gear.
My motto has become: race like a madman uphill not downhill