I PULLED INTO A PARKING SPOT. IT WAS SLOPED BACKWARD. THE CAR WAS STOPPED COMPLETELY, AND SILENT. I WAS DISTRACTED BY A MINOR TASK, AND THEN EXITED THE CAR, INADVERTENTLY WITH THE POWER ON AND THE TRANSMISSION IN DRIVE, NOT PARK. WHEN I NOTICED THAT I COULD NOT LOCK THE CAR, AND REALIZED THAT THE CAR WAS STILL ENERGIZED, I REACHED OVER AND PRESSED THE ON BUTTON TO TURN-OFF THE CAR-- WHICH IT DID. AT WHICH POINT THE CARD IMMEDIATELY HEADED BACKWARDS, DOWNHILL, CROSSED A STREET WHILE I WAS TRYING TO SLOW THE CAR FROM OUTSIDE THE DRIVERS DOOR, AND FINALLY HIT A LAMP POST AND STOPPED WITH DAMAGE TO THE REAR BUMPER. I TRIED TO REPRODUCE THE ISSUE, WITH THE SAME RESULT: THE CAR CAN APPEAR TO BE OFF AND IN PARK, WHEN ON AN UPWARD SLOPE, BUT WITH THE CAR ACTUALLY ENERGIZED AND IN DRIVE. AND THERE IS NOTHING TO PREVENT ONE FROM TURNING OFF THE CAR, AT WHICH POINT THERE IS NOTHING TO KEEP THE CAR FROM ROLLING BACKWARD. AS A COMPARISON, A COLLEAGUE HAD A CHRYSLER PACIFICA PLUG-IN HYBRID, WE TRIED THE SAME AND THE CAR WILL DEMAND TO BE PUT INTO PARK BEFORE RE-ENERGIZING. THUS, THIS SEEMS TO BE A TOYOTA-SPECIFIC HAZARD. I WAS VERY LUCKY THAT NO OTHER CARS WERE INVOLVED. I TWISTED MY KNEE TRYING DESPERATELY TO KEEP THE CAR FROM ROLLING-AWAY, BUT THE STRAIN WAS NOT NOTICEABLE BY THE NEXT DAY.