I lost a small trailer I had owned for many years once.
The coupler was attached; I had unloaded it and driven 25 miles.
The safety chains were attached.
What I did not do, was put a bolt, or a lock in the coupler. Which was something I always do, because I had heard of couplers coming loose from that.
I had crossed some rough railroad tracks about a minute before, which apparently managed to get the coupler to come partially off.
When I started up a hill, I felt the trailer jerk, and head a bang. When I tried to brake, the trailer repeatedly smashed into the back of the tow vehicle, and then bounced back to the limit of the safety chains. The "S" hooks on the safety chains gave out after about 6 of these jolts, and the trailer thankfully hit the curb on the right side of the road, and exited the road.
Since then, I never tow with a bolt, or a lock on the coupler, and I use screw closing quick links on the safety chains, so they cannot come off.