I was on my way to buy safety chains and install them on my friends utility trailer I borrowed as there were none on it. Stopped to turn left but there was a small bump in road right before it. Felt a light slam into back of my truck, got out cursing that somebody hit me. Nope, trailer was 5 feet back, lock was in coupling still closed, thought it wasn't adjusted and slipped off ball. Nope, nut unscrewed from the ball shank on draw bar. Ball was still in coupling but no nut/lock washer to be found. I know it was torqued prior I had used same setup many times, no clue how it happened, towed other stuff for decades prior. I don't remember if somebody had borrowed it or not. New drawbar's and ball now get Locktite, drilled roll pin, and paint marked. I recheck that mark each time I install and each time I stop for anything.
I was lucky! It was a 30 mph road, I was in a turning lane and stopping, only damage was a straight slice punch in plastic bumper cover. Next day was 1500 lbs and highway speeds. Could have been much worse.