The biggest thing is the horse busting out of the barn at a full gallop. Not only's it dangerous because you might run someone over, but it's very bad to put that much stress on a horse's musculo-skeletal system without any warming up, especially if they've been in a stall for a long time. It's just asking for a bow or strain. Plus, it's really bad manners. ;)

And then the bit with Saturn greeting the mare, that's dangerous in case the stud gets it into his brain she needs some sexy-time, since he can go through the fence and injure himself and if the mare's not receptive she can do injury to him. Those were the only two that really jumped to mind for me.