One of the most common problems I've found in dealing with horses that have already been through one trainer is that they are seldom, if ever, ridden outside an arena/pen. For that reason, those trainers don't realize how much they depend on the fence to control the horse they are riding.
For example, I'm currently riding a 4 year old mule that had been to another trainer, had 50 saddlings, and had never seen the outside of a roundpen. That is also a perfect example of the trainer and owner not communicating enough to come to a happy conclusion for both of them. The owner was wanting a nice, broke mule that he could take out and enjoy trail riding on. The trainer, in spite of having rode the mule 50 times, he never once took the mule anywhere even remotely resembling a trail, he spent so much time on teaching her to move around with her nose tucked up to her chest in a correction bit (which he started using on her 3rd ride...but that's another story) that she has a mouth about as soft as a cinderblock, has no idea what "stop" means, and has been taught very well that she doesn't have to follow her nose....so she's perfectly happy to lope along with her nose pointing right and her body going left. Now, that wouldn't be such a horrible thing if it was trained as a counterbending exercise like you see in reining horses, but this mule also has no idea what leg cues mean so there is no controlling her body with your legs.
Another thing I've noticed is that any time she gets even remotely stressed, which is almost constantly, she starts grinding her teeth. That's another symptom of having been ridden by someone who wanted her "bridled up" before she was ready for it...and as a result, even slightly picking up pressure with the reins results in instant stress to her.
Because she has always depended on a fence to "keep her in a circle", she has no idea how to obey the commands of the rider on her back to maintain her direction. I've been dealing with a very dangerous situation with this mule. When she first came here, she had no idea what lateral flexion was. Her neck was about as stiff as a steel pipe. I've fixed that and basically got her listening well at the trot and walk but she has a panic attack when it comes time to lope, especially if I ask for a circle. She'll go along okay for half or maybe 3/4 of the circle and then she'll decide that she wants to go the other way...so in spite of me picking up on the inside rein to try to correct her direction, she continues to lope off in the direction she wanted to go.
And, to top it all off, she's barn sour so she always knows which direction the barn is and that's the way she wants to lope sideways toward. I truly feel sorry for this poor mule every time I mess with her. She's been jacked around and screwed with and micromanaged so much that she honestly has no idea how to just be happy under saddle. Even now, she can't sit still for more than a minute or so before she starts stressing about what's about to happen so much that she basically panics and begins either backing up or walking off forward or sideways. I've been working with her and she's getting better but there is still a very long way to go.
Update: The first part of this post was written about a year ago and then I just sort of forgot about it until now.
I sent this mule home about 30 days after writing the previous part. It took a lot of time and a lot of patience, but she finally clicked onto the fact that being ridden could be fun. I just took her a few thousand miles and whenever she would follow her shoulder, I'd just gently bump with outside leg and lift on the inside rein until she straightened out again. I learned that she responded badly to having anyone get after her and increase the pressure....so I didn't. I'd keep the pressure steady until she responded.
At first, it would take up to a couple hundred yards of travelling crooked and sideways, but it steadily got shorter and shorter. One day, she just quit doing it altogether. After that, circles were basically a breeze. The first time I asked for a canter on a circle, she had a momentary flashback and started to get tense and noodle-y through her body, but I just kept my energy down and kept light pressure on and she quickly relaxed and was loping calm circles in just a few minutes. I was really happy with her when I sent her home.