Car was parked on driveway (6 degree slope to rear) Car was placed in park Driver exited car to go into house when she came out car had rolled down the driveway to hit another car parked across street rear facing bot cars had rear damage. The same thing happened to the car one week later when parked in the garage placed in park but started to roll back. Was able to stop it because I was still in the car I use the parking brake now all the time