Just saw this on mr2oc.com, and I thought I would share because it's quite different from your usual story of a jack failing or something. He was working on his pickup truck. The car was on the ground (high enough that he didn't need to jack it up), and he was fiddling with the shifter linkage from under the car. He took it out of park, his handbrake wasn't on, and the car ran him over. He was working on it himself and had to drag himself out and into his house to call the ambulance. He's lucky he wasn't stuck under the car with noone around.
Some nice, graphic pics to put a bit of fear into you. Not for the squeemish.
http://s335.photobucket.com/albums/m466 ... 0Accident/
Goes to show that even when your car isn't jacked up off the ground it can still be dangerous - always think about safety when you're planning to work on your car, having personally had the displeasure of breaking some bones due to being lax with safety when working on a car, I can tell you the time lost due to being in hospital and/or incapacitated for a couple of months greatly outweighs the time taken to ensure you're safe and your car is probably supported/restrained. This has certainly reminded me that I should be more careful about chocking my front wheels when I jack the rear off the ground.