My worst one was three years ago. It was just before I sold my horse to go to uni, and whilst I warned the new owners about my fall it wasn't the reason I sold him!!
He was always a bit spooky, and had a tendency to bolt and buck if he didn't like the situation he was in. It was something I mostly trained out of him, but the reaction was still there when things went wrong. Of course, it went wrong one day when out hacking in the middle of the hills with my sister: she was riding a youngster, who spooked at a rabbit and then tripped and nearly fell over. This freaked my horse out, who couldn't decide whether the rabbit or the other horse falling was more scary, so he went to bolt.
Normally I would have caught him before he got going and it would've been fine, but I'd injured my hand the previous week and was riding one handed with long reins, so when he bolted I wasn't quick enough to stop him. Usually, when he got going, the best thing to do was to do a quick flying dismount, as he would start to buck and work himself up, and would bronc until you hit the deck (and as an ID he had a LOT of power). However, I decided that I would stick the bucks out that day, and while he bolted across a nice grassy field I stuck on really well, despite his best efforts.
However, he then took off across a rough bit of ground, where there had been an old drystone wall that had long since fallen down. He realised how rough it was, and skidded from flat out to a standstill right in front of a pile of drystone rubble, throwing in a massive buck as he did so, and spun 360 degrees. I went flying off into the remains of the granite wall, headfirst, and cracked my hard hat down the middle.
I was unconscious for about 2 hours, waking up in hospital after my X-rays. They had originally thought my neck was broken, thankfully it wasn't, but I was kept in hospital for several days for brain scans, as I had a severe concussion and they wondered if I had brain damage. Thankfully I didn't, I was lucky beyond belief, but I scared a lot of people, and even my university wanted proof I wasn't brain damaged before I could attend. Scary time - especially for my little sister, who went to fetch help (no phone signal on a Scottish mountainside) and got me airlifted to hospital, and who caught my horse and took them both back to the yard and looked after them. She was only 14 at the time, and had originally thought I was dead when I didn't move after she heard the big 'crack' of my hat splitting.
But yeah - that's why I get a bit funny when I see people riding without hats. It literally saved my life: my head would've fared much worse against granite rock, especially if my hat cracked in two!