Well, I guess I have two that I would count as worst falls for me. The first I can't entirely count as user error, because I was very young (8 years old) and didn't know what I was doing. I was riding my cousin's horse, a normally docile older arab mare, around their barn in circles. My aunt got bored and went into the house and left me to it. I don't remember all the circumstances, but they stallion they had in one of the paddocks started jumping around and squealing, I guess the mare I rode was in heat, anyways she jumped up to give a sideways kick in his general direction, and I flew off her, and landed in a pile of landscaping bricks my aunt had dug up. Broke my wrist, collarbone, and had to have reconstruction on my cheekbone/eyesocket. Thankfully I have little memory of it all.
My worst fall since actually knowing what I was doing was about 4 years ago. Pretty much completely my fault. I was working at a trail-riding facility where we gave guided trail rides, and I did the finishing work on a lot of the young horses they raised up to be string horses. At the time I was working a half wild halflinger pony, and I decided to go out with a group ride. The group going out at the time was a bunch of girl scouts, and I figured if I kept the pony back enough from the group it would be a good chance for her to just follow and learn the trails. Most of the ride went fine, the pony was nervous as hell but responding well to me and seeming to just take it all in. As the ride came closer to finishing, the pony got more uppity as we came in sight of the barn, but I thought it was fine, she was just a pony for goodness sake.
Well, about 100 yards from the barn front, she bolted on me and ran up alongside the trail group, I got her head around and got her circling, and she happily circled for a minute or so, I was laughing about it to the trail guide when wham! She planted her front feet. Having been "leaning into the turn" while she was spinning, centrifugal force just flung me off, and the pony snorted and hightailed it to the barn. I stood up cussing a blue streak and looked into the faces of 12 grinning girlscouts. Nothing was hurt but my pride, in that circumstance, but being thrown from an itty bitty pony infront of a crowd sure topped my life's embarrassing moments.
I gave up dignity when I got horses though, right?
Ah, thank goodness for horses teaching us humility in the most embarrassing ways they can devise.