What are the most common handler mistakes that make trailer loading harder than it needs to be?

The most common handler mistakes in trailer loading are well-documented by Clinton Anderson, Pat Parelli, Warwick Schiller, and virtually every natural horsemanship trainer, and most of them fall into a small number of predictable categories. The single most damaging mistake is releasing pressure when the horse pulls back. If a handler is applying pressure to ask a horse forward into the trailer and the horse pulls back — and the handler releases the pressure in response — the handler has just taught the horse that pulling back removes pressure. This one mistake can create months of resistance with a single repetition because horses learn from the release, not from the pressure, and they learn extremely fast. The second major mistake is standing in the trailer trying to pull the horse in with a lead rope. This positions the handler ahead of the horse's drive line, removes the handler's ability to apply meaningful pressure from behind, and often results in the handler being pulled down when the horse scrambles backward. Anderson specifically trains handlers never to position themselves in this way. The third mistake is quitting after progress stops. Handlers who stop a session when the horse is at two feet in the trailer, because they are encouraged by the progress, often find the next session starts over from the beginning. Parelli and Anderson both teach finishing each session at the highest point of progress achieved that day, even if it takes additional time. Warwick Schiller adds a less-discussed mistake: working the horse when it is too stressed to learn. Handlers who apply pressure to a horse that is already above its learning threshold simply add stress to stress, and the horse associates more negative emotion with the trailer rather than less. Schiller recommends recognizing when a horse needs to simply stand and decompress near the trailer rather than be actively trained.