No. 1 sin against horses: pushing them too fast, too hard. A shocking number of people are still in the 1800's in regard to horses, considering them things, work tools, to be used like a shovel or a hoe (don't call my mare no hoe!), and if they break under the stress of the work, they are thrown away in favor of new ones. And you will even see this from people who claim "natural horsemanship" without having a clue as to what that even is.