When I was a beginner I was tacking my first horse up and wrapped his lead rope around the handle of the barn door to keep him from wandering around. Can you guess what happened? He set back and almost pulled the barn door off the hinges! The top hinge actually started coming loose and I had to have my Dad fix it (I was boarding at the time). Boy, was that ever an important lesson. I was so grateful the horse didn't actually pull the door completely off and panic.
Another time I went to look at a horse I was interesting in buying and when I drove over to the house the horse was haltered with a long lead rope attached to a tree while the horse grazed. When I drove up the driveway the horse panicked and set back. Luckily the horse didn't get hurt. The owners should have known better than to tie the horse like that as they supposedly had horses for a very long time.
Another time I was letting my very first horse eat a little grass in the yard where I was boarding and he put his foot inside a ladder that was laying on the ground. It happened so quick and I didn't invision him doing that. When he drew his foot back and the ladder moved he almost panicked but luckily his foot came loose. Potential for disaster there for sure!
I think most new owners do dumb stuff like that. And then after a while you get a feel for what is safe around horses and what is not. But that knowledge often comes from the school of hard knocks. Heck, I had an incident with a gate a while back that was flat out stupid on my part. But I won't get into that right now.