dear oh dear... I can only imagine my own reaction if I saw my car rolling off heading for the wall!
I must say it's something I've almost done myself, but not on an incline like that. With the handbrake being auto "most of the time", you tend to forget if you've switched it off manually. If you are just moving the car around in your driveway for example and not wearing the seatbelt, the handbrake won't auto-engage when you open the door....