Never had any formal training, but I've yet to be unseated, and I've gotten into some pretty harrowing trail rides.
The last that sticks out clearly was on a family vacation.
Me, my then boyfriend (never been on a horse before), my stepdad, and a close family friend took a guided trail ride through a nature preserve.
Part of the trail took us close to the backside of a trailer park, where some kids were playing.
On the trail was a deer and two fawns, the kids were throwing rocks and spooked the deer off, one fawn followed her, but the other panicked and ran at us, right under my horse's belly which of course spooked him.
The horse reared and spun, slamming into the horse behind me, pinning my leg pretty good, and Loren, the family friend, had my mount's bit smacked pretty hard into his elbow.
No one was hurt, and the horses calmed pretty quickly, but mine was done basically.
He was jumpy and his bit had dug in good when he smacked the other horse.
Didn't want his mouth raw, so I let him have the bit after I moved him up behind the lead mare.