I don't feel the same as Hound, I'v always been happy with every horse I bought. But that's because there were no surprises. I knew just what they were. Vet check, xrays, and being around the horse a little so I know what it's like. Solves a lot of problems.
A lot of that is picking appropriately. The steady eddie type, I got at the time I needed a horse like that. The hot little ferrari, I got a little later, LOL. A LOT later. And most people will never be suited to a horse like that.
If you ask any trainer, in any discipline from reining to dressage to showjumping to rodeo, the biggest and most frequent mistake people make is getting more horse than they can handle. Too green, too young, too spirited. Any trainer, no matter what kind of riding will tell you that people often think they can handle a green horse, a horse out of work that needs to be brought back, a horse with a problem.
ESPECIALLY green. People always seem to think it's SO easy to train a baby. If it was, why would all those professionals be getting paid to do it?
And then, of course, they don't want or can't afford lessons or a trainer. And training a horse is nowhere near as easy as it looks.
I'd say that getting a horse that's too much is just about as common as getting stuck with a lame/unhealthy horse. But I'd also say, the two are just about neck and neck, and both are very common.
Most problems aren't as easy to fix as it might seem. A seller saying a problem is easy to fix - don't count on it. Much of what horses do 'wrong', they do because of how they are built, or how their brain works, you don't change that - ever. And habits - habits are the easiest acquired and the hardest lost, in a horse. With a horse, the word 'Habit' should be proceeded by the words, 'Cast in cement'. Something a horse has 'acquired' can be the toughest thing to eradicate.
I've seen sellers pull a lot of tricks. Don't EVER take to heart, ANYTHING the seller says about the horse's history or how much he WUVS him or anything like that. Stick to facts, figure out what the horse is right now, and forget the 'sales shpiel'.
Probably the sneakiest one I ever saw, was the seller who told the buyer she couldn't handle the horse, but the BUYER, the buyer and the horse were meant for each other, thank HEAVEN they found each other. When the buyer trotted the horse over a pole on the ground, the seller burst into tears and sobbed, 'I could NEVER get him to do that!! You and he were MEANT to be together!!!'
The buyer was so flattered at the complements to her skill that she bought it hook, line and sinker - AND bought the horse, and spent the next years struggling with a spoiled, ornery young horse that she could not handle any better than the seller.
The goofiest thing anyone ever did to ME was let my two dogs out of my car and tell them to run at me on a green, just broke youngster. When asked why, the dealer said, to 'show you how well behaved the horse is'. When the one dog(six months old and 75 lbs) had its hind leg in my left stirrup and was licking my face, was about when things got interesting.