THE VEHICLE WAS PARKED AT A GAS PUMP. AFTER FILLING THE GAS TANK. I STARTED THE ENGINE, LEAVING IT IN PARK. I STEPPED OUT OF THE VEHICLE TO CHECK MY GAS CAP AND THE VEHICLE STARTED MOVING BACKWARDS QUITE FAST. I RAN IT DOWN, OPENED THE DOOR, BUT BEFORE I COULD BREAK IT THE DOOR WAS SERIOUSLY BENT. I BELIEVE THE ONLY EXPLANATION IS A FAULT IN THE TRANSMISSION WHERE THE GEAR SLIPPED FROM PARK TO REVERSE.