I was working on my car today which I normally keep on a set of Harbor Freight dollies I bought when I was building it.
I rolled it out and reached in from the outside and turned the key to start it and it fired up but was making a weird rumbling sound. I blipped the throttle a couple times with no change and glanced in the cockpit and noticed the transmission appeared to be in first and the speedometer was running at 5 - 10 mph. I thought what the heck - reached in and shut the motor off, pushed the stick into neutral and re-started it and everything sounded normal. The parking brake wasn't on either so I couldn't figure out what was going on. Then I noticed a little pile of rubber at the back end of the rear dollies and realized the car had been running in gear on the dollies like it was on a chassis dyno.
Fortunately, other that a bit of rubbed off paint on the dollies where the tires were slightly in contact - no damage done. Stupid mistake but I'm impressed the car didn't climb off the dollies and that the rollers on them kept up with the tires.