I was riding with a friend across the street from my barn. Her 16+hand OTTB spooked and she came off. He was running for the street, and my green horse (with vision issues) was dead set on following him. I knew there was no way my fat 15 hand Tennessee Walking horse could catch her horse and stop him before he hit the traffic, and I also didn't know at that point if she was okay. I had to make the hard choice to emergency dismount my horse and get control of him to stay with my friend and make sure she was okay, her horse unfortunately didn't make it.
To this day she is upset that I didn't go after her horse, but I knew I'd never catch him and that even if I did on my green horse I wouldn't have been able to safely stop both horses before the highway. I hope I never am in a situation like that again, but feel that I would make the same decision.
"Putting pads on a gaited horse is like putting a mask on a beautiful woman."