Game Version number: 1.021.169
Modded?:** (No)

For this issue if a vehicle is not at a complete stand still it will not correctly switch to stationary physics if there is any kind of force acting on it. So doing something like bringing the Skoda 105 to a rapid stop, putting on the handbrake in neutral and exiting the vehicle (also works when staying inside) while on a slight incline will cause it to slide down the hill.

In order to prevent this you need to wait for the game physics to register the vehicle as stationary (so wait for the suspension to settle once stopped and no other G-forces acting on the car) and then putting the handbrake on and being in neutral will not make the car slide.

This ends up being an issue in multiplayer games when trying to stop or dismount vehicles quickly, can be quite annoying to have your supply truck slide 1km down a hill when you've jumped out to attack an enemy ambush.