My biggest mistake was in learning not to tie a horse directly to a (unattached) trailer without baler twine or anything. I'd heard that you're not supposed to tie to an unattached trailer, but knowing what I do now, I wouldn't tie directly to an attached trailer either without baler twine. I didn't usually tie on there but all my tack was going in the trailer tack room after a ride so I thought I'd quickly tie him to the trailer, untack, then bring him back out to his pasture. He was being a fart dancing around and pawing because his friend went straight back up to the barn, and when I came around the side of the trailer I yelled "HEY!" and clapped my hands to get him to quit pawing. Thank goodness he's virtually bombproof and isn't a puller, because when I came around the side I startled him, and he backed up a couple steps and threw his head up - enough force to rock the trailer backwards and pull the jack off the blocks. It landed with a booming crash, and the saintly horse just stood there. That's when I actually considered the physics of it - horse tied high, meaning any pull from the horse is going to pull near the top of the trailer, and the trailer is only about double what the horse weighs - and it's on wheels. Wheel blocks or not, this is a stupid idea. Could have been much, much worse, particularly if it was a different horse or if he had pulled the trailer to the side rather than backwards. Lesson learned!!!