I've been reading this thread for a hunnert and fiddy years now, but now I've gone and done it.
Volunteered to pull my sons' Scout troop's 16' box trailer. I've towed it several times before.
I have my son change out the stinger, from the 2" that was mounted to the 12-5/16" that was required. The guy who usually tows it comes strolling up with, "Oh, changing out your ball, huh?" I say, "Yeah, had the 2" in there and have to change to the 2-5/16"." "Oh no," he says, "you need the 2"." "No, that trailer is a 2-5/16" hitch," I say. "Oh no," says he, "it's a 2" for sure. I ought to know, I towed it here this morning."
I questioned my own memory and listened to him. So my son puts the 2" stinger back in. I back up to the trailer and the other guy does the hook-up. Before we pull out a little while later, I take my boys around to the back, and I explain that we do a double-check before we leave, even if we trust the guy that did the hooking up. I figure it's a good lesson for the boys. We check the chains...secured and crossed. We check the brake release...connected and an appropriate length. We check the electrical connection...right pins and securely plugged and not dragging and enough length not to pull out. We check the hitch itself...locked closed and pinned over the ball. We check the stinger install...pinned and keyed. We checked how level the double axle trailer looked, and discussed whether we should switch to a stinger with a different drop height or not...not on this pull. We checked that the tongue jack was cranked up, rotated and pinned in place. With each item we discussed the function/necessity and consequences of getting it wrong.
What's the one thing I didn't think to check with my kids? The ball-to-hitch size relationship. Because the ball was already hidden under the hitch. My eyes even hesitated on the embossed 2-5/16" numbers near the hitch safety pin, but my mind didn't click.
So about half a mile down the road, over a bump, the hitch jumps off the ball. I feel the jerk as the tongue drops down into the chains. That's when my mind clicked. I knew what has happened before the words OH CRAP could escape my mouth. Fortunately we were only doing about 40-mph at that point, the road was straight and just very slightly uphill, and although it was wobbly and nerve wracking, I eased off the gas and we gently coasted down to a safe stop on the shoulder with no damage to ourselves (except maybe my underwear) or anybody else. Spent a good little while talking to the boys about what went wrong and what went right in the situation. I think it was an even better lesson that they - and I - will not soon forget.
I almost started cussing that "usual" guy, but stopped myself. I explained to my kids that it was my fault 100% for not verifying EVERYTHING before driving off. I told them that this had been the extent of their teaching on this subject, and that in future it was their responsibility to always help me check EVERYTHING about our trailer hook-ups. I've got near-future lesson plans for them that will include load weight distribution and load securing, concepts of load versus GVWR, braking functionality/logic, etc. I imagine this thread will still be going a few years from now when they're old enough to drive, and I don't want their pictures to end up here!
