Chap I know on the 300ZX forum has a MK4 Astra SRi as a daily runner and this happened yesterday.
No one was injured. Can't say Im bothered about the Mini, but shame about the Astra.
He parked the car as normal, reversed up to the brick wall of a secure car park. Above the wall is a grass verge going up to another car park. Basically the Mini has gone across the car park, down the verge, hit the brick wall, landed on the car and then rolled onto its roof.
He nearly took his Z to work, so is relieved in that respect.
The chap who owns the Mini claims the hand brake was fully and the car had slid down the hill or the brakes had failed.