Not too long ago I picked up my new boat (not new of course) about 45 miles from where I lived. As I was pulling off the interstate close to home and slowing, POP, and off the ball the trailer goes. I was down to about 30 or so and managed to use the rear bumper to stop the trailer and control it off the ramp itself. Quick inspection showed that not only had the coupler popped off the ball but the safety chains had failed as well. The only thing still hooked up was the wiring. Besides the trailer jack, no real damage to anything else fortunately. And fortunately it didn't happen at 75 on the interstate.
So I let the PO hook up the trailer after he guided me back to it. I know I did a look as I walked by and it looked fine and locked down but it was at best a cursory inspection. I *think* the coupler was sitting on top of the ball and not fully engage regardless of the lock position. I know there's a rule about trailering that I'm violating there. I've tried to get the coupler to pop since then and have been unable to do so, so my only assumption is it was user error as I neglected to check someone else's hookup.
So I'll be replacing chains with stouter of course, completely resetting the coupler so I know it's adjusted right, and I also ordered (and just received) a trailer guardian, found it on ebay for about $25. Trailer Guardian Video
It looks to be a pretty solid little unit, heavy steel and comes with a locking pin so it also doubles as security. I'll be installing it and using it in the future, seems to be pretty cheap insurance vs. what could have happened. Unfortunately there's no way for me to really give it a review unless something bad happens, but initial inspection it looks like a good item.