As we get more experience, we are perhaps more able to see when trainers do things that could potentially overstrain or damage a horse. Not all horses will come up lame ridden front to back, or inverted, or overjumped, or ridden into the wall to teach a western rollback, etc. But some will. I certainly know more than one horse that was permanently retired or pts because of chronic stress injuries incurred while just still doing Training Level dressage. Others soldier on, with clear musclulature or NQR problems but not actually to the point of being euthanized.
I think that a good training plan should be conservative and not put any physical risk on the horse. However if you send a horse out for 30 or 60 days with the requirement the horse should have mastered some particular skill, the trainer may well need to push more than is ideal.
I am still curious what exercise to straighten a horse is this potentially dangerous, and what the injury actually was. I tend to think of straightening exercises as lateral work at the walk, which is pretty safe.
I do tend to think that a lot of apparently acute injuries, are really chronic stress injuries that build slowly until they are noticed, at which point the people start looking for an acute cause (he must have cast himself in his stall, or twisted something in turnout, etc) that isn’t really there at all.