I've been thrown and fallen off more times than I can't count on one hand, but I've only had a couple of really scary falls.
The first one was when I just started riding years ago... I was on a gigantic Thoroughbred mare (her name was Legs, go figure xD). She was way out of my experience level and my instructor knew I was afraid of her, but she put me on her anyways. In short, the horse spooked and I forgot myself, screamed, grabbed onto her with my heels and she took off. I ended up falling on my head. One of the stalls you could get into from the paddock was open and if I hadn't fallen off, the level of the roofing would have nearly decapitated me. My helmet cracked clean down the middle and I got a concussion, but otherwise, I was fine. Scared the heck out of me though.
My second fall was on my guy, J.R. My friend and I had these trails we would go on right off the highway (nowhere to ride, so we had to ride to these places) and it was this place where the footing was primarily clay (not a good idea <.<). WELL. It had rained a few days before and there was a big collection of water and my friend and I decided we'd race through it. Stupid idea, but...we were being idiots. ANYWAYS, I took off on J.R. and it was all going well and we were splashing about, but then his hoof got sucked into the wet clay and we flipped. I don't remember the actual fall, but I remember landing on my back in freezing cold water and trying to scramble away because I didn't want to get clobbered by a 1000 lb thrashing giant xD The scariest part of that fall, though, was that he began running for home, but thank God, he stopped and let my friend catch him before he reached the highway. That was possibly the scariest moment of my life x_x.
NEEDLESS TO SAY, such shenanigans are very well avoided now xD