AFTER DRIVING INTO CONDO GARAGE, I PUT TRANSMISSION INTO PARK, WITH ENGINE RUNNING, AND OPENED DOOR TO EXIT VEHICLE - CAR STARTED MOVING BACKWARDS - THE SURFACE IS FLAT. I WAS PARTWAY OUT OF THE VEHICLE, HIT THE BRAKE PEDAL BUT COULD NOT STOP IT BEFORE THE OPEN DOOR IMPACTED A CONCRETE PILLAR INCURRING DAMAGE PREVENTING DOOR FROM BEING SUBSEQUENTLY CLOSED. IF I HAD BEEN FURTHER OUT OF THE CAR THE RESULT COULD HAVE BEEN FAR WORSE. THE VEHICLE BEHAVIOR WAS EXTRA ALARMING BECAUSE MY EXPERIENCE PRIOR WAS THAT THE TRANSMISSION TENDED TO BOUNCE BACK TO PARK WHEN SELECTING REVERSE. ALSO WHEN IN REVERSE - EXTRA EFFORT IS REQUIRED ON THE GAS PEDAL TO MAKE THE VEHICLE MOVE. SO HERE - NOT ONLY DID IT SEEM TO BE IN REVERSE DESPITE MY PUSHING TO PARK - THE VEHICLE WAS AT IDLE SPEED WHICH HAS PREVIOUSLY NOT BEEN SUFFICIENT FOR MOVEMENT.