It was around 1984 I had a 1980 Toyota 4x4 P/U that I used to helped a friend move into a house on top of a hill. I had backed into the driveway to unload the last load, and left it in reverse without parking brake set. Went outside after a few beers and truck wasn't there... it was sitting at the bottom of the hill with the nose resting against a chain link fence. What we figured out is someone knocked it out of gear getting beer out of truck. Fortunately, no damage or injuries. From that time on, I set parking brake Everytime I park, regardless where, even in my shop with automatic transmission.
Anhoo, some lessons are easy and some expensive.
Cheers, Ron