My closest call to date (I've been miraculously lucky, and never seriously injured).
I was visiting my dad when I was about 13 or so. He lived on this ranch-thing. Picture about 20 houses around a large circle driveway in the middle of a few hundred acre "ranch". The landlord (money grubber) had decided that for extra income he'd buy some horses cheap and give trail rides on them.
When I came to visit, he had 1 he had bought thus far. I knew nothing of this horse (or the landlord) but I was under the impression that this was on of those horses that kinda trudges along trails nose-to-tail (typical of trail riding places that give rides to random kids and such). Not too exciting, but I was told I could go ride the mare if I wanted to. (Of course I wanted to)
What I didn't know- He had bought the mare at auction, knew nothing of her, just assumed she was broke, but no one there knew how to ride a horse, and they had never ridden her, nor did they ever (at that point) even take her out of her paddock, and aside from getting her in and out of the trailer, had never even lead her.
Again, I didn't know these things beforehand, and didn't ask either.
So, I got her, I saddled her, and I got on. No issues, she seemed alright.
I opened gate, I walked thru gate, shut gate, and carried on to explore the land on horseback.
About 200' out of the gate, mare exploded. Not the little pitches and head tossing- she turned into a world-champion bronc. All fours off the ground, kicking, rearing and spinning and going completely nuts.
I bailed (one of my first lessons- and the only lesson I really remember- is the emergency evacuation). Not very graceful, I landed face down in the mud- I try to spin over on my back and get out of the way, and before I know it, I have 2 back hooves on either side of my head- one so close that she gave be a bloody nose bringing it back up (I was kinda sideways- was like being punched in the face).
That was, by far, my closest call (that even topped when my old horse decided to flip himself off the side of a bridge- with me on him).
And that, also, was the only horse I never got back on.
(She later became a BROODMARE.. ??!!??)