I have drilled this in my sons heads and still recheck the connection, pin and safety chains every time they hook up the trailers. About 20 yrs ago, my father was up visiting and we were standing at the end of my driveway saying goodbye and a Ranger bass boat on a trailer came sailing across the end of the drive apron and missed my dad by about 5'. It glanced off one of my neighbors trees ( destroying the side of the bow) and stopped when the tongue hit the block foundation under his living room window. The guy driving was towing his buddy's boat and wasn't used to pulling anything. The hitch lifted off the ball, no pin and he didn't hook up the chains.
I also had a long time customer that was hit by a large enclosed trailer that came unhitched and crossed the median, hitting him nearly dead on at 55mph....it came very close to killing him and his wife. The end result was that the guy pulling the trailer and the ones hit both ended up losing their businesses. The liability bankrupted the first and injuries left the victims unable to work again.
At a retirement seminar last year, the insurance advisor there said that in Illinois the average settlement for a death caused by negligent drivers now averages $750,000. I don't know if that's accurate or not but I did purchase a 2 million umbrella policy that covers liability on the home, rental property and vehicles soon after that.