There are a couple from my childhood mare, who had a bit of an issue tripping. I wasn't the smartest teen apparently because I still used her for all sorts of stuff. Once I was jumping her and she tripped going over the jump, I hit the ground and looked up to see her falling towards me, apparently she saw me or something because she twisted and hit the fence instead of me.
Another time she tripped and came down, trapping my leg under her body, she started to roll towards me to get up and I started kicking at her with my untrapped leg until she rolled the other way. All in all she probably fell with me or on me something like 15 or so times, somehow I was never seriously injured.
This one always sticks in my head too. I was riding my lesson horse in my new saddle which was super slippery, apparently whatever I had used to clean it was a little too conditioning.
He saw some fake flowers outside of the fence and leapt sideways to escape, normally I could stick those type of leaps on him, but with the saddle, I sort of just slipped right off the side. Somehow I ended up underneath him and had maintained a hold on my reins too. He freaked out even further and started rearing repeatedly with his hooves on either side of my head. Once my trainer's yells got through to me I let go and he bolted away.
My most recent is with my current horse. It was about two years after I bought him, 6 months after I had moved him to my parent's house. I knew he was herd bound and it was spring so I figured it would be a little worse than usual. We went out on the big loop ride I usually do, I thought that I would take care of the issue in one good wearing ride. Man was I wrong. The entire ride was spent circling, doing serpentines, leg yeilds, etc, etc. He was jigging, spooking at everything under the sun, and completely lathered. After something like two hours we were coming to the end and he had settled a little, but not a lot, my body was done so we continued the loop to get home. The last mile is along the highway and as we came down it there was a lawn mower parked on our side for sale. On the other side of the highway were two pick ups parked. He immediately went into even higher alert and started freaking out, I had him just barely on the right side of control and was doing everything I could to get him calmed. He settled a little bit and we started past the lawn mower, something freaked him out again and he tried to bolt into the highway, which just happened to be occupied at the moment by a semi truck coming in one direction and a big pick up in the other. I was not even two seconds from jumping off and letting him kill himself, but I managed to get him stopped. After that we got past the lawnmower, the rest of the way down the road, and back to the house. I still get nervy when I have to ride on the highway now and that was like 5 years ago. He's thankfully not nearly as spooky or herd/barn sour as he was back then and I just put him on SmartCalm which has worked wonders to get rid of the worst of his edge.
Yes, I am totally aware that all of my terrifying moments are due to my own enormous stupidity or hubris.