Nut backed off ball caused accident

This very same issue happened to an employee of mine this Summer- Lost his boat and trailer on the way to Havasu. Fortunately the chains held or it would have been a lot worse.

I always use a lock washer and tighten in a vice. The other thing I do is grease the ball (just takes a little bit) every time I hitch up. When I unhitch I wipe it off. Way too many people (most) don't grease the ball ever. Big mistake!