I had an incident on Tuesday of this week. I was hauling my 5x10 utility trailer back to work when I looked in the rear view mirror and saw the trailer standing up. I slowed down and the safety chains pulled the tongue down. I was lucky as the trailer went under my bumper and nothing was hurt.
When I hooked up the trailer at my house, I gave the trailer the conventional pullup/shake test, connected my safety chains, and checked the lights. Standard procedure, nothing out of the ordinary.
After investigation, the trailer did not come off the ball but the ball came off the bumper. The washer, lock washer, and nut were missing from the ball. The ball was still in the trailer coupler. No stripped threads, just the nut missing.
I now have a locknut and pin system to make sure the nut does not come off. I was lucky this time and someone was looking out for me. Thank goodness I did not have my teardrop connected.
Moral of the story. Check everything, even the things which seem unlikely!