A friend told me a story about his car that I felt compelled to share so the same mistake isn't repeated.
My friend, let's call him Fred, was loading his car up on his trailer after a car show. He got the car on the trailer and put the car in park while he walked behind it to collect the ramps. Just as he was setting up to strap the car in, he noticed his friend, lets call him Bob, circling around for a parking spot. The street they were on was pretty busy and the only empty spaces were in front of and behind Fred's trailer. Fred offered to move the truck up so Bob could park. During the five feet the truck was idling forward to clear space for Bob, the car managed to pop out of park, and slide off the back of the trailer.
The only thing that kept the car from being totaled, is that it hit the front of the car parked behind him, luckily a friend of his.
So the lesson is, never move your truck, car, or trailer for a friend. No, just kidding, always strap your car down before moving it. At least chock the wheels until you can get to your straps.