Had a VERY similar experience. I'm scarred for life lol.I was hanging out at my Dads house in the early 2000's, my Dad had just bought a Z4 and I guess came inside to read the manual since it had so many features. A while later he's screaming to see which one of us kids (I was an adult) F'd with his car... He forget to set the parking brake, somehow the car got enough momentum from the semi-flat driveway to roll up a few feet of uphill slope, then a few hundred feet down a hill through scrub oak and into his newly installed fishing pond. He ended up getting a body kit which insurance installed and painted for "free".
That situation always sits in my mind, to the point I will wake up and run downstairs to check my parking brake sometimes, it's happened to me for almost 20 years regardless of which manual car I've owned.