First fall: during a lesson in winter in an indoor arena. I had stopped and was totally daydreaming while my instructor was taking a phone call. Snow fell from the roof, horse spooked sideway... no more horse under me... rider on the ground.
Instructor said: your first fall? When you reach seven, you're a real rider!
Lesson learned: don't daydream while on a horse!
Worst fall (I had a hard time choosing...)
In winter, during a snow storm in an indoor arena, end of private lesson on a friend's older morgan that didn't like to canter. Arena space is very small, dark, a young boarder decides to gallop where we are riding, two dogs are fighting in the middle of the ring. And the instructor is insisting that I canter my horses once more before we quit. The horse is annoyed and I'm tired.
I was cantering on the right lead towards the wall, the horse saw the other horse cantering towards us and veered left at the last minute. I flew on the wall. Instructor says: "you're fine, do you want to get back on?" as I'm in a heap in the sand, trying to catch my wind and wondering if every limb is still moving. No thanks, just driving home was torture. Had 2 broken ribs.
My bad falls were almost all on unknown horses, with little known instructors. Each time I KNEW something wasn't right before it happened and was too shy to say so. I learned to speak up, for my own safety because instructors aren't sitting on that horse, but I am and should trust what I feel.