#1) Thinking the victory was getting the horse into the trailer (and not realizing she would crouch flat and scrape all the skin off her spine backing out from under the butt rope).
#2) Not realizing that exiting out of the exit door on a trailer might give the horse a bright idea too.
#3) Teaching horses that if they won't go forward, they have to back up instead. This gave me a couple of horses that when they were scared or confused by an obstacle on the trail, would start backing as fast as they could. It was fun when we backed into a whole line of horses, and it was also fun when we backed toward the edge of a cliff. What has worked better for me is to circle the horse instead. This also keeps them from getting out of work when they won't go forward but is less dangerous when the horse does it suddenly on the trail.
#4) When a horse would not accept the bit in her mouth, I tied her to a fence to try to hold her head to where I could reach it. I know, BAD idea. When said horse felt trapped, she pulled off the fence board and it chased her around for awhile. I learned no matter how many baby steps it takes, teach the horse to accept the bit without force.
#5) Long-lining without getting the horse used to the lines properly first. The horse spun in circles, wrapping herself in the lines and very nearly wrapped up a couple of nearby riders as she galloped away from the lines that were chasing her around the arena.
#6) Thinking if I run into a problem it has to be fixed before I quit for the day or the horse will "win" and we'll have problems forever. So many things have popped up one time and if I ignored the problem just that one day, it never came back again. But before I realized this I spent a lot of exhausting hours trying to work horses through problems. One day doing figure eights with a horse at the canter, this horse would NOT turn to the right. Every time we reached that point she would flip to the left when I asked her to go right. I rode the pattern for an hour (poor horse) with no success before going home frustrated that my horse had "won." The next day I braced myself to try again. My horse had no issues whatsoever, did the figure-eight perfectly every time I asked. She just hadn't wanted to turn right that day. It wasn't a big deal to her, just to me.