I tied Okie to the trailor (when I still had it) awhile back and went to the barn to get something. I turned around and he was following me. He popped the clip in two. I've mentioned If I leave him too long with a slip knot he'll try to untie himself. I have to keep it up high because he will try to use his foot. He hardly moves once he's loose. I think he just likes undoing them. When he was sitting back at the wash rack I would wrap the rope several times. One boarder was fussing because she said if he tries to sit back I should undo him and should be able to untie him and always carry a knife. To me letting him go would defeat the purpose and let him know he could get loose. I figured if he fell he could get up and would just let him tire himself out. It worked because he doesn't do it to me anymore but was that being mean to him? The ferrier didn't turn him loose either and Okie was raising his legs for him before he quit.
If he is a horse that will pull till he breaks something, I wouldn't leave him unattended at something like that bar at the wash rack, and I wouldn't assume if he sat back to the point of falling that he could get up. The warning to not leave him unattended and always be able to untie him is advice I'd also give. Yes, if he fell at the wash rack, he might be able to get back up, but if he has scraped off a bunch of hide from his legs on the concrete in the process and needs the vet, it is a pretty darn expensive lesson, and some down time for recovery. Tieing to something low like that wash rack, and on slippery footing like concrete, is a fairly dangerous situation. They can get quite a bit of leverage tied low, and may bend that rack bar right over or sit back on their hocks and fall.