This is a rather extreme example, but I'll tell it anyway. Girl at the barn I used to ride at was an eventer. Her idea of "warming up" was to ride the ring for 5-10 minutes at a hand gallop before putting her horse over 4' jumps. None of us agreed with what she was doing, and her new horse was lame inside of a month.
Now, whether this was from the lack of warm-up or just the fact that she was pushing his program too fast is debatable. From a mental standpoint, I do not think she was doing her horses any favors: when she rode them on a regular basis, they were over-sensitive and explosive, both on the ground and in the saddle.