Mine is probably when I was about 7 and I went horseback riding with my cousins and two leaders, one in the front, one in the back. And my horse was the last one, other than the guide's horse. Something spoked his horse and he was knocked off, and that spooked my horse, so my horse took off and ran into the street that we were about to cross and ran right in front of a car, wasnt hit though.
I used to take lessons, not too long before that, but they never told you what to do when your horse gets freaked out.