I actually had that happen when pulling a boat once. It was a new to me boat (not my first), and it didn't grab the ball as good as my old one - but I thought it was just the differences in boats. Luckily it popped off on a back road about a mile from my house and I was only going about 25mph. Still scared the crap out of me - but I handled it good, just gradually slowed to a stop so it didn't roll under my truck and have my boat rear end me. I now jerk up and down on the trailer when hooking up to make sure it is locked down on the ball. That could have ended up bad if I would have made it to the highway.