Let's be charitable and say that their handbrake's fucked and it rolled back onto the road.
Best I've seen was a car parked completely across the sole entryway to a local car park. Absolutely no way for other cars to get in or out. Never seen a traffic warden look so perplexed.
Best I've been told about was at a place I worked at years ago. They had a garage next door and apparently the mechanics used to be b*****ds for parking vehicles across our driveway when they had a backlog. Despite being asked nicely, it kept happening. So, after a particularly stressful day, one of our drivers returned to base to find a car blocking his path. He stomped into the garage, only to be told the car would be moved "when we're ready". Said driver comes back out fuming, and angrily tries to tip the car over! The rest of the staff watch him for a minute before thinking "aye, fair enough", and help him get the job done. Everyone then gets finished up and scarpers quick before the mechanics come out and find their customer's car spinning on its roof in the middle of the street