After I put the four wheels back on the wagon and set it down, it started to roll. I learned a new value of steel-toe boots when working on cars - wheel chocks. Got away with that one because the car had only moved a couple of inches. Might have broken my ankle if I waited a split-second longer. Now, the funny part was - I was working alone - how do I go get a real chock? Unlace the boot and hop over to where the chock is sitting and hop back to the other wheel, shove the car forward and chock it to recover my boot...
Not to make light of your situation, but I echo Liquidh8's observation that we're all grateful that the damage did not extend to your awesome paint body work.