I had my dad's mustang on 12"x12"x12" aluminum blocks on the side of the house on plywood sheets. I went to jack the rear end up to turn the wheels and in the process of lowering it back down, the car rolled backwards and the front fell down. The damage is minor, just a little kink in the front bumper, but I felt bad of course. Lesson learned, don't ever assume the ground is level. I guess I was on a slight incline and it was enough to roll. Learn from my mistake and always have a way to keep the wheels from turning or use jackstands.