For me, it was a broken hip coming off of a 4 yr old 17hh that I broke to ride - he had learned to buck off another trainer and she gave up on him, so never was saddle broke. When I got him, I got him with that lovely bad habit. Persistence paid off and I had him going in about three months and was trailering him to state parks and camping and riding with groups and alone. Then one day he pulled a fast one on me out of the blue and I got slammed down on my side. Usually I'm up and on my feet and grabbing the reins in about half a second. This time I tried to jump up and about tossed my cookies. IT HURT.
After a few days immobile and then a few days on crutches and tramadol, I worked with him some more but only in my arena and only with a bucking strap to keep him from getting his head down too low just in case. I was scared of this horse. Just a few weeks later when I felt he was solid enough to go on with someone else, I sold him to some folks who wanted a novice eventer and he's doing GREAT. I disclosed everything to the new folks and he did try a thing or two with them, but they enjoyed him so much they just kept on and he's settled right in.
And nowadays to be honest, anytime my own riding horses throw out a crow hop or get fritzy on a cool spring day, I get a little bit tense and admit I think I will forever be a bit of a scaredy cat when it comes to bucking. I'd been training horses for over 20 years and have been thru a lot of crazy stuff. But that one horse put the 'fear' in me....I don't know why it was this particular one, but so be it! I DO NOT train horses that tend to buck hard like that anymore. Just can't do it..