You've clearly never witnessed a '76 Mercury Cougar roll backward down a hill, hearing nothing but CLACK CLACK CLACK CLACK coming from the parking pawl until it reached a spot near the bottom where it could catch. I did. It was my brother-in-law's car, parked in front of his mother's house. This is the same hill my mother's '72 Monte Carlo slid down on a snowy, icy day, with parking brake set. Yes, it was steep.
You guys who get on here and claim things are impossible need to step just a few more feet out your front doors occasionally to see the real world...not the one you're seeing when you look at the computer. It's great for you that you've never had it happen, but that doesn't make it impossible. I'm guessing you're the kind of guy who will argue with a fence post. If so, please ignore my posts. Block me, even, because I'm not interested in hearing about how you think something I've seen with my own two eyes is a myth.