I had this happen once. I was only going a short distance with an empty 4x8 flatbed, so I did not bother putting the lock on the coupler. I had used this trailer for many years without any incident. So, there was no concern.
Shortly after going over some railroad tracks, all you know what, broke loose, and so did the trailer. After several very hard jolts, the hooks on the safety chains gave out and the trailer became liberated. Luckily, it went to the right, rather than the left, and on coming traffic. As I watched helplessly in the mirror, I saw something I never expected see, It was kind of surreal, watching the trailer jump the curb and become airborne!
To my credit, the total damages were minimal, to my well built trailer. However, the damage to my comfort level when towing, was substantial.
I will NEVER, EVER, tow anything without either a lock or at the very least a nut and bolt in the lock hole of the coupler. I also replaced my "S" hooks with big quick links, that screw closed, and can not come open.
Also, make sure you do the one thing I did do right, cross the safety chains under the coupler, to support the tongue, during hitch failure.