My title also includes the phrase “just when to jump off.” I feel that under most situations you should remain mounted and attempt to calm and control a misbehaving or panicky horse and avoid trying to do an emergency dismount. This is because there is often a greater risk of injury dismounting than staying with the horse. When you are riding, the motion of the horse and the wanderings of your own mind can lull you into complacency. However, as I mentioned before, your mount is never complacent and the next thing you know you have a problem of some kind that has caught you unaware. Usually a horse will give you a warning that it is about to freak. It will tense and its ears and eyes will focus on the object of its anxiety. Often a gentle pat and a few calming words are enough to defuse the situation. If things are past that point and your horse flees, you often can turn and stop a horse once you are at a safe distance from whatever spooked it. However, there are situations when the only recourse is an emergency dismount, and your brain needs to be able to rapidly calculate the odds favoring one action over the other given the particular situation.
One clue to the correct course of action is to decide if you still have any elements of control over the horse. We were once riding on our own property when a neighbor unexpectedly shot off a Estes model rocket. It flew up in the air with a loud swooshing sound that spooked my horse. I could tell that he had totally tuned me out and was not responding to any to my efforts or signals to calm and reassure him. I was able to stop him enough to hop off and then lead him back to the barn where he finally felt safe and calmed down. I was certain that if I stayed on him and worked at controlling him at some point, I would have been thrown and injured. An additional problem with jumping off is that it may result in a loose and panicking horse that is a danger to himself and others. In my situation with the model rocket I still held the reins and had control over his movements.
I have spoken a lot about the many advantages of trail riding in the company of others. Horses are reassured by being with other horses, although you need to judge their compatibility and be careful about spacing them to avoid kicking or biting. If things do go wrong you will have other people to help you out. We have a female friend who was approached by a strange man in a deserted horse trailer parking area and was relieved when an another trailer pulled in and the man suddenly cleared out. My wife and I always ride together and I believe it is much safer that way.
There is also the occasional carelessness that we humans are prone to. I would feel rather foolish admitting that I tried to back a horse off of a trailer who was still tethered at the head except that Jane Smiley confessed to once making the same error in her book “A Year at the Races.” I have also left gates open, failed to properly secure a trailer tailgate and made other kinds of careless errors over the years. The only thing I can say is that once something like this happens I try not to allow it to happen again by devising a system to guard against it. In the case of the trailer tailgate, I now secure the latch with a clip and verbally say "trailer secured" to my wife each time I do it. To err is human, but to err over and over is just plain stupid. I have been lucky in that my mistakes have resulted in inconveniences and not injuries.
When I say do not ride too close to the banks of streams, I say this because they are sometimes undermined and will give away under the weight of rider and mount. This has happened twice to me. The first time my horse landed on all four feet in shallow water and was mostly upset to find himself suddenly separated from his companions. The second time was very scary. A tree had fallen and blocked a narrow pass between a steep hillside and a stream. As I tried to work my way around it, the bank suddenly gave way, and I found my horse with his belly on the ground and left legs folded under him. He was supported to his right by a small sapling and his right legs dangled over the edge. Below us was a tangle of branches from the fallen tree. If the sapling gave way we would fall into the tangle and I could imagine both of us being injured or worse as a result. I decided to roll off his left shoulder and hug the bank as much as possible. He then pushed himself up with his left legs and hopped to the left side away from the bank. I was worried that he might unintentionally trample me, but he avoided doing that and came to rest with all four legs on the ground. I think I was more shaken by the experience than he.
I mentioned barn grip as a good thing to carry in a tow vehicle during the winter. Barn grip is crushed limestone sold in feed stores to provide better footing around barns during icy conditions. One winter we had total cabin fever and during a break in the weather decided to visit a favorite riding area. We pulled into the parking area unaware that the surface had melted, refrozen and was a solid sheet of ice. It was not obvious to us until we pulled in. The trailer jackknifed and the wheels spun even in four-wheel drive. The lot sloped dangerously downhill toward a pond. I got out and slipping and sliding flung barn grip in front of the wheels as I scurried ahead of my wife who was behind the wheel. In no time we were off the ice and back on the road home, another disaster averted.