Hmmmm... I don't really have a crash story regarding MTB's. But I do have one to share going back to around 1976 when I was a teenager, riding my Raleigh 10 speed.
I was out riding with my best friend/next door neighbor. We'd always take off and just...
leave...you know? We'd both tell our mom's "See you later! I'll be home for dinner", and would ride 20 - 25 miles or more during the summer months.
Well that one fateful day, it had rained the day before, but was okay and the roads were dry. There were a few puddles on the sides of the roads though. We were cruising along and my best friend laughed at me, as I veered off to the side and went through a puddle and got my bike wet, my brakes wet, and put a wet stripe up my back. I slowed down and let him go ahead and then raced to catch up. Just up ahead, we descended down a WICKED steep hill. I'm talking so steep, that they'd probably close the road in the wintertime or heavily sand/salt it, so people could get up it. Anyway, my best friend whipped down the hill and then slammed on his brakes at the bottom at the sharp T intersection and turned left and stopped. I came flying down the steep hill too, but, my brakes were still soaking wet and my Raleigh 10 speed couldn't stop. You can just guess what happened next....
(Now remember, this was the 70's... No helmets, no anything...)To this very day, all these decades later, all I remember is my Raleigh 10 speed trying to make that left turn with me dragging my left foot like a motorcross/dirt bike racer. Next thing I remember, I woke up being wheeled into the ER of the hospital with a neck brace on and my left hand all bandaged up and a huge bandage around my head!
My best friend said that I went off the road, went airborne like Evel Kinevel, and me and my Raleigh 10 speed crashed into the woods and my bike went head-on into a tree!

What saved my life is that my left hand got caught in the handbrake, pulling me over the handlebars to the left side and not into the tree. I was told I dislocated my left thumb, ripped the webbing between my thumb and index finger of my left hand, and when I went over the handlebars, my head landed on a big rock, knocking me out cold, and caused a massive gash in my head. My best friend ran to the house nextdoor and the family all came running out and dragged me out of the woods and called the ambulance and called my parents.
Damage assessment:
8 stitches in my left hand.
My left thumb popped back in place and all bandaged up.
13 stitches in my head and all bandaged up.
A foam neck collar due to a mildly sprained neck.
A stay in the hospital (just overnight for observation).
A totally trashed, twisted and bent, Raleigh 10 speed bike.