Not an mtb, but watched a Vespa scooter approaching in the mirror a few months back, it was wet, first rain for a few weeks so greasy roads, rider wasn't wearing any gloves, and had an open face helmet on. Crossed my mind how painful it would be if he came off.
Passed me and he decided to stay out on the wrong side of the white lines for a few more cars before pulling in for oncoming traffic. Touched the front brake, front wheel washed out and he was on the deck…bike slid down the road with a sickening scrape and the rider managed to pick himself up and limp to the side of the road. Face appeared OK but hands were obviously causing him some pain!
On the same section of road I have seen two cyclists collide, one riding down the nearside on a folding bike, one on the offside. Folding bike changes sides in front of my van, doesn't look and collects the other bike as he pulls out. It always scares me that peds will cross in front of my van without checking for motorbikes overtaking me. More than once I've had to give them a blast with the horn, makes them jump out of their skin but saves the motorbike riders from a spill 🙂
Last one…watched my younger brother descend a steep ramp into a car park with no hands…has done it many times but this time he removed his feet as well…in the time it took me to wince, he was off! He didn't come off too well…cuts and grazes everywhere!