I hooked up the truck to my current borrowed trailer, an 18' econo-trailer open deck, last night in preparation for taking my car to fix the clutch. Truck was almost totally unloaded and the entire rig was on a slight to moderate downhill. I pull the car out and begin to load it. As soon as the rear tires of the car got up on the ramps suddenly the whole rig is heading down the hill. Uhhh, "holy $#!+ what's going on?". I'm not really sure at that point but I throw the car in reverse and back off the trailer. When the back tires are back on the ground everything stops again.
Once I'm sure nothing is moving I get out of the car and check out the situation. Apparently the weight of the car on the back of the trailer was enough to lift the back tires of the truck enough that the tires skidded. I had the parking brake set and the truck in park. Luckily the truck was pointed in a "good" direction and it didn't hit anything during its ~15' trip.
I would never have thought that the weight of my Camaro on the back of the trailer would have been enough to lift the rear of a 2500HD extended cab pickup truck. Obviously I was wrong and thank goodness it wasn't any worse than it was. Looks like the only casualty was the trailer ramps. They were shot up into the trailer about 1.5 times as far as they should go and my neighbors were treated to the sound of me banging them back out with a Ponycar Wheelwell Clearancing Tool (aka handheld sledge). I had to pry off the little angle iron ramp-stays to get them back out, too.
As a funny side note, I talked to my Brother-in-law after it happened to let him know I'd either be late or not make it. I told him why and he chucked, said he'd done the same thing at his house. Doesn't mean I'm not dumb, just means I'm not the only one who is dumb.
So, moral of the story, chock the trailer or the front wheels of the tow vehicle if you are on a hill. I certainly will from now on.