I ride English, so I don't have any absolutely terrifying trail stories, but the camp I go to has some amazing trails, and they've given me some interesting stories. These are all from last summer. :P
I was in the advanced riding class last summer, so we're given all the young, spooky, touchy horses to ride. We call them our SPED (special ed :P) horses.
Anyway. I was riding one of them, a young dappled mare named Skye. Her main problem was bolting whenever she had an open space in front of her. We were riding on this gorgeous trail, with a pretty steep hill on one side, and a giant drop off on the other, when we came to a hill that was probably a good 55, 60 degree angle- you couldn't see the bottom of it. The horses were all pretty jumpy, side-stepping, tossing their heads, etc. I have no idea why I thought it would work, and I thank god it did- I gave Skye her head (something I would never dream of doing, even in an enclosed ring), and let her pick her way down. I was shaking so bad, I don't know how I managed to stay on. I just kept picturing Skye bolting, me having no reins, and both of us tumbling over the edge of the cliff. (She did manage to run me into a tree branch though. :P)
Another time, I was on a young, very spooky mare named Buttercup (the horse in my picture. :]). Our trail leader was one of the barn directors, whom I love to death. I was riding right behind her, and she was on another young, spooky mare (she always chose the worst leads, :P). Anyway, Elise (the counselor) liked to take us on the trails that weren't actually trails. Anyway, Buttercup spooked at a tractor before we even left the barn, and then decided to skitter, side-step, and bunny hop across the road. We finally made it to the trails, and Elise turned off onto an itty-bitty trail, with thorn bushes on either side. While the rest of us are sitting there getting poked to no end by the thorns, Elise was just sitting there humming.
Each time we turned off onto a new trail, I would jokingly (kind of :P) berate Elise for her choice of a trail. Finally, she decided to let me pick a trail. I pointed to the left, because it wasn't as overgrown as the trail on the right, and seemed like a better choice.
Definately not a better choice. Not only did it end up being the same trail I went on two years ago, where we ended up having to gallop madly back to the barn because of the presence of ground bees (thankfully they had moved on by last summer), but it turned into...well, not a trail. Towards the end, the grass with well above the horse's heads (they were having a field day :P), there were tons of fallen logs to go over/around, and there was a cliff on one side, and a hill on the other (like many of their trails are). Finally, we managed to see the edge of a field. However, the only way to get to it was by walking through a thick stand of trees. Elise being Elise, she gives her young, spooky horse her head, squeezes her forward...and closes her eyes. (She's a character. :P) Buttercup found a way around the trees, but it turns out that the field was an unmown hayfield...needless to say, we were about 20 minutes late getting back to camp. (They were getting ready to send out a search party. xD)
Not so much a terrifying trail, but it was...interesting nonetheless.
There was a trail however, that I wasn't on, but scared me either way. Myself and some others were sitting by the ring across the road, where you can see parts of the trails. Long story short, we saw one of our counselors walking towards us holding two horses, and our first thought was that someone had been bucked off or something. We got pretty scared when we saw the ambulence pulling into camp though. Thankfully, it wasn't that bad- one of the girls had gotten stung by a bee, which she's alergic to, and used her epi-pen like she was supposed to. They called the ambulence to make sure she was okay, but it was still rather terrifying before we knew what had actually happened.
"To ride a horse is to ride the sky"~ Author Unknown