I watched a nice 70 Roadrunner roll off a trailer in a parking lot. The guy had put the car on the trailer, never tied it down or even put the car in park. They had put away the ramps though... and walked away. It didn't seem to do much damage.
Another time, we sold my son's Saab and the people came with a trailer to pick it up. They parked the trailer out on the road and didn't want to back into my driveway. That bothered me, it's safer to do it in my driveway, but I figured I wasn't tying it down, so whatever. Then they seemed afraid to load it. Since I'd been putting cars on trailers for a long time, I jumped in and drove it up.
As I started up the ramps, for some reason both of them decided to step over between the back of the truck and the trailer. This wasn't the safest place, but I guess they thought they needed to direct me on. I'm glad I saw them do this...
As the car went up the ramps, suddenly the trailer came off the ball and the nose of the trailer shot up about chin level and almost hit them... They had driven 80 miles and had never thrown the lever over on the coupler. Luckily, I had been paying some attention and was watching them. I stopped the car immediately with the trailer kind of balanced at that point. If I'd driven any farther, I would have hurt them both.
They got out of the way and I backed down... very slowly and it fell back on the ball. I said "WTF?" and once the trailer was secured on the ball, I pulled it back onto the trailer.
At this point, I really didn't want anything more to do with this... I didn't want to know how they strapped it down or anything... Plausible deniability.... I pointed them in the direction to go home and went inside. Last I saw them, they were backing down the street to turn around in the intersection rather than going the way I suggested.