Here we find my Roadster.

It’s parked on a slope, facing downhill. You’d want to stop it rolling away then. Thing is, it has an automated manual gearbox, so no Park. Instead, the recommended procedure is to put it in reverse before stopping the engine (not after, because the gearbox doesn’t work without the ignition on) as reverse is the lowest gear available and thus the least likely to be able to turn the engine. So I did, and took my foot of the brake. Nothing happened for a moment and then the car succeeded in getting one cylinder past compression and moving downhill a little, whereupon it succeeded with the next one and moved a little more.

Moral of the story: put the handbrake on unless it’s a really gentle slope.