Leaving a ratchet and socket in the engine bay and having them drop out the bottom of it and almost destroy the windshield of the car behind me... at 60mph. Lucky it missed :L Also knew someone who took their car to the mechanic around 40 miles away cause he's their go to, with a funny noise. Turned out the wheel's weren't on right and he drove 40miles of motorway with the wheels not on right. Hilarious now, since no one died and all.. haha