In over 30 years of riding, there have been a few memorable ones... Mostly the early years. I've only been separated from my horse twice in the last 20+ years.
The first was at 8 yrs old on my first pony, 3 days after I got him. He knew voice commands, and I was cantering him on a dirt/gravel road at my Grandparent's farm. My Mom thought he was getting away from me and yelled "WHOA" from across the yard. Yep. He stopped on a dime and left change. I sailed across his head and landed on my right shoulder, breaking my colllarbone. Thanks, Mom!
I've been bucked off green horses, and had a horse rear up and fall over backwards with me, almost falling on top of me... but she fell on the fence and landed right next to me instead.
I was riding one of my Saddlebred show horses next to a wooden rail fence (he was a rail hugger) when my jeans got caught in a split in the old wood rail, pulling me off the horse and suspending me in the air and breaking the board off the fence, spooking my horse. I managed to hang on to him, though! My jeans had to be cut to get me loose from the board, though...