I hate to sound like an ass but stuff like this is why I was taught to always put on the emergency brake before getting out of a vehicle.
Way back in highschool, I got to borrow my dads prized car for one day. I parked it in the HS parking lot and put the parking break on. Well... it failed and my dad's fancy ride (Yes. pontiac fieros were once pretty fancy to me) decided to roll across the parking lot and re-park itself into my friends old civic (thank goodness it was a friends).
Now I always am OCD about putting the parking brake on plus I always give the car a shake and a push once I get out of it to make sure its not going anywhere.