A horse named Shanghai was hobbled (his legs were tied together so that he couldn't move) and left on the ground for three hours because he was too energetic for his rider. Afterward, in order to hide his rope burns for filming, his legs were covered with makeup and hair. Hobbling is an outright violation of the American Humane Association's (AHA) guidelines.
One horse was killed and another horse was injured after being placed with two highly strung geldings, despite concerns that the geldings would be too aggressive.
Another horse was killed after falling off an embankment in a severely crowded paddock.
When the horses were moved to the stables, another horse died after being fed large amounts of food that he wasn't used to. The horse had shown signs of colic, an extremely painful illness.
When the horses were moved back to the paddocks after this incident, another horse had the skin and muscles of her leg torn away by wire fencing.
To read the rest of the incidents and what can be done in the future, you can click here to go to PETA. What I agree with PETA about is that with all the CGI and everything that can be done with computers it would seem like the risks for animals should be lessened and not increased and there is no way such a significant number of animals should be killed. Think about what happened on HBO's Luck and how fast that was shut down even with just a fraction of the number of deaths.