People at the front of a group of hillwalkers/horse riders/etc. who shoot ahead and have zero feckin' clue what's happening at the back of the group. When hiking, it's often me at the back, as I'm not a very fast walker, and I get into a fury when the speed demons at the front disappear. It drives me nuts when I eventually catch up to them, usually scurrying and breathless, and then they immediately take off again!
I use to have a fast horse, but now I have a slow one, so it's usually me at the back of the group. We were trotting up a hill today and I was the tail end of a group of four. Two were warmbloods, so they were a lot faster than my little Highland. I was trailing by a ways, but he was happy at his pace. Then he spooked at a roadworks sign. Slammed on the brakes. Tried to spin. We had a discussion. Everyone else is just disappearing into the distance, totally oblivious to me having a mare. Leg on, urge horse past the scary sign, then pick up the trot again. Now I'm miles behind. At which point I realize I didn't tighten my girth enough (I'd tacked up in a rush) and my saddle is sliding sideways. F*ck, I think. I ask horse to walk, which he does, very cooperatively given his pals are far away now, and try to straighten it as best I can. I needed to get off, straighten it, and tighten the girth, but felt there's only so much I can expect of my little green horse and didn't fancy hanging onto him in the middle of the road while futzing with the saddle, with the horses out of sight.
The others stopped at a junction where you turn onto another road, so we caught up to them, but I was not in a good mood. Then one of the WB riders, who was in a rush to get back, takes off in trot again (on a road that goes slightly downhill, so I never trot it) and disappears. The others stayed in walk, but if I'd been on Gypsum, I would have been screwed. Add that to general bad mood. Highland was fine, but it's the Principle of the Thing, dammit. She didn't even know he would be fine because she didn't ask.
It's just one of those things that really pushes my buttons.
I also want to add that when I had my fast horse and was usually at the front for trot/canter work, I always had one eye on the horses behind me. If someone suddenly stopped trotting or disappeared out of my sight, I pulled my horse up.