Parked my car outside my mates house (on a fairly steep hill) and put the hand brake on. The car stayed still whilst I got out of the car and whilst I was waiting for my mate to open the door (because I was watching it!).
When I came out 10mins later my car had rolled 20ft backwards down the hill, onto a neighbours lawn and into the corner of their house! I have a dent in the boot and a crease in the bumper!!
The car must have rolled fairly slowly as when I got to it the boot opened/closed fine and the boot landing panel (or whatever its called) was fine. The handbrake was still on too.
Thankfully it missed any parked cars and didnt cause any damage to the house. God knows how much its going to cost me though....