I PARKED MY V-6 AUTO TRANSMISSION MUSTANG ON A SLIGHT INCLINE AND EXITED THE VEHICLE. APPROX 60 SECONDS AFTER EXITING, THE MUSTANG STARTED TO ROLL DOWN THE HILL. I WAS ABLE TO CATCH UP TO IT, DIVE IN HEAD FIRST AND PUSH TRANSMISSION GEAR SELECTOR FORWARD TO MAKE SURE IT WAS IN PARK(IT WAS). IT CONTINUED TO ROLL DOWN HILL UNTIL IT IMPACTED A BUILDING. AFTER IMPACT I CONFIRMED SHIFT LEVER LOCKED IN PARK. THE TRANSMISSION DID NOT LOCK DRIVETRAIN AND THERE WERE NO SKID OR SLIDE MARKS FROM REAR WHEEL LOCKUP. DAYS LATER, I RETURNED TO SCENE AND RECREATED INCIDENT, PARKING IN SAME LOCATION. SAME ANGLE, ETC. PLACING THE CAR IN PARK HELD IT'S POSITION. PLACING THE CAR IN ANY OTHER GEARSHIFT POSITION AND RELEASING THE BRAKE PEDAL CAUSED AN IMMEDIATE ROLL BACK. THERE IS ABSOLUTELY NO WAY I EXITED THIS VEHICLE UNLESS THE VEHICLE WAS IN PARK, IT WOULD OF ROLLED IMMEDIATELY AND THE DOOR WOULD OF KNOCKED ME OVER. MY SUMMARY IS: PARK SELECTION OF GEARSHIFT LEVER FAILED TO LOCK DRIVETRAIN, ALLOWING CAR TO ROLL DOWN HILL AND IMPACT A BUILDING. *TR