Been there, done that!
Had one of my Audis up on a jack working on the exhaust. Got everything bolted up and wanted to see how it sounded. Reached in and started it up. Bad mistake. Front wheel drive. Stick shift. Back wheels up in the air. It took off, the rear axle beam bounced off the jack, the back end came down on the jack and reversed to floor curvature of the trunk floor. I was able to catch up to the car within about 3 or 4 feet and reach in and turn it off before it took off up the driveway and out into the street. Luckily I had the emergency brake on, so it slowed it way down once the back wheels finally got on the ground.
30 plus years of working on cars, and that's the stupidist thing I've ever done! No real damage done. I was able to pound the trunk floor back down to pretty much how it was. (It was pushed up a good 6 inches or so) Most damage was to my ego!
