The most noteable example I can think of a horse not matching was a woman at my old barn, she was petite, about 5', probably 60 years old. She was an extremely timid woman, and an extremely timid rider.
Her horse was a four year old 17+ hh oldenburg, very beautiful, very interesting color (gray, but with chestnut flicks in a fleabitten pattern, I have no idea what it'd be called) very hot, very spooky. He definitely needed an advanced/confident rider. Even us barn help were intimidated just bringing him in from the pasture. He regularly reared, tried to bolt and spooked at the same things everyday, the barn door, the horses in the stalls, children, rocks, dogs, insects, the person leading him, etc. He wasn't mean though, thank goodness. No bitting or striking out, just a very nervous animal. If you went to his field, he would come right up to you and enjoy some petting.
She fell from him almost every time she rode him. The BO ended up riding him much more than she did. Eventually she took the horse to a different barn (I got to be the lucky one that trailered him! BO didn't have a big enough trailer) and I actually think she ended up doing much better with him with her new trainer. She came back one time to do a show and the horse was surprisingly calm and she looked happy. So at least the story has a happy ending (maybe?). I have no idea what they are doing now though.
She spent a lot of money both on him and on training fee's and board fee's at my old barn, he was there for at least two years not making much progress, both on the ground and being ridden. Plus all those falls couldn't have been good for her. It was definitely a case of being over mounted. But she loved the horse I guess!
Thinking about it now, I guess I do know quite a few riders who for whatever reason were not happy with their horses. Interestingly enough, those who had horses that didn't fit what they wanted to do (jumping, dressage, w/e) were very quick to sell their horses, but people who were over mounted hung on to that horse.
Another memorable girl- my former best friend who boarded my horses on occasion. She started out with a big gray quarter horse, but she didn't want him because she didn't like his color. Sold him and bought a dun yearling. Sent her off to training and when she got back, decided she didn't want a dun, she wanted to show on the appaloosa circuit. Sold the dun, bought an appaloosa. Had the horse in training for a few months, and then decided she wanted the dun back. Sold the appaloosa and now doesn't ride at all. When I talked to her regularly, she would still complain that she would have been amazing if she'd kept her dun horse. I'm not sure what she was looking for in a horse, it's bizarre to me that someone wouldn't like a horse just because of it's color.
This turned out to be quite long, so I'll end it now.