| personally i haven't done too much damage (yet). i drive a chevy aveo so getting my 6x10 cab into the backseat was always an adventure, eventually i got it down to a system, but i put a few dents into the door frame the first couple of times.
i had a guitarist in a band i used to play in who would stick his 4x12 behind the back seat of a minivan and one night he was driving home and had to step on the breaks hard, rocking the cab forward and and then on the recoil it smashed out the back window.
another guitarist i know had a 75 strat that he backed over with his car. he was doing some cleaning in his garage, propped it and a few other guitars against his car, then realized he had to move his car, so he hops in, puts it in reverse, and somehow manages to knock the strat under the front tires. snapped the neck like a twig. | 
