I've mentioned this before, when Ranger the police dog ran over a lady with his owner's truck. It really amazes me how many people think that putting their car in gear is enough to keep it from driving or rolling away.
Well, today my daughter told me of her adventure after work. No, she wasn't the non-parking-brake offender. She had turned onto the street from her work parking lot and where the street curved around to the left, there was a car kind of in the way, heading across the curve toward the guardrail. Every now and then it would move forward a bit and then stop again.
My daughter looked through the rear window, and the car had no driver, so she parked along the curb behind it and put on her flashers. After she got out and approached the car, a glance through the driver's window revealed a parking brake that was not set.
The doors were locked, but the back window was down far enough that she could reach in and unlock it. She looked around and saw a guy pretty far away walking toward her, so she signalled to him but got no response. The car moved forward again, prompting her to unlock the doors, climb in, and put on the brake. Then she got back out and locked the doors.
That's when the guy showed up and said, "Can I help you?" It was his car, and he said he'd parked it way back there. She told him what she'd done and then continued on her way. We don't know if he learned his lesson about using his parking brake, or if he just got annoyed instead at the nerve of someone getting into his car.
It seems to me I've told the story before about the car I saw rolling across campus when I was in college. It may not happen very often, but cars can move when the parking brake isn't set.
Again I say, use your parking brake!