I see this stuff on a daily basis, and honestly a lot of the time it no longer surprises me.
What I see most commonly: wrong ball size for the trailer. Probably seen it 200 times in the last 30 years. 1 7/8" ball, trailer requires 2". No problem right? It pulls just fine. When you go to load your tractor, it pops off of the ball, the chains catch, and guess what happens next? Your $60,000+ pick up truck's tailgate gets trashed. I watched it happen today on one of those GMC trucks with the multi purpose tailgate things. Destroyed the tailgate. Absolutely destroyed it. I was told by someone I know that works at the parts dept at the GMC dealer that those tailgates are about $6000 currently. Something else I see all the time is the latch not latched. Just drop it on the ball and drive off. Chains dragging, wires dragging (if there are any wires). See TONS of them running around with NO trailer lights, and I mean none. Not non-working lights, no lights whatsoever. Another funny one was last week when guy loaded his L3901 that was in for 50 hour service, guy was trying to straddle a hole in the floor of the trailer and, well, it made another hole. Rear right wheel fell right through. Guy was real lucky he didn't come off of the tractor. Ya know...when you have a HOLE IN THE WOOD FLOOR, you just might want to consider replacing the floor sometime in the next 20 years.
Seen another one a few weeks ago, guy shows up pulling an L2501 with a BH77 BH and a LA525 FEL--pulled by a 4 runner. The trailer was a single 3500 lb axle with those little lightweight boat trailer tires. That trailer was screaming for help. The tops of the fenders were BURNT from the tires rubbing on it. I asked the ol' boy how the 4 runner pulled it. He said it did ok, but was really underpowered. Well with the fender ON the tires, I would imagine it felt like the brakes were on. DUH! That wasn't even the best. The best part was that the tractor didn't fit on the trailer with the loader down, so he had it hauled with the FEL bucket as far up as it would go and the BH swung sideways. The front tires were right up against the front rail and the rear tires were maybe (if you're lucky) a 1/2" on the trailer. I told him we'd deliver it back to him for his trouble. Honestly I was really worried he'd try to haul it back home on that same trailer which was dangerous as could be!
Just when you think you have seen it all, someone surprises you.
The sad part? All my trailers have working lights, up to date tags & registrations, maintained bearings and axles and springs (and brakes), yet I guarantee I'll be "that one" who'll have some sort of problem the next time I pull somewhere....wasn't there some guy named Murphy that used to be popular?