I had that happen once... Parked in a buddies driveway which was so ever slightly on a slope. Car wasn't moving when I got out or while I stood there a minute, so I went inside. I came out about two hours later and my car was gone... Slid down his driveway, across the road, and into the neighbors yard and all I could see was a tree behind it. I was like oh shit... Went over there and I was about 2 inches from sliding into the tree.
It was a close call...