VEHICLE WAS BACKED INTO A RESIDENTIAL GARAGE. STARTED VEHICLE AND PULLED FORWARD OUT OF GARAGE. PLACED VEHICLE IN PARK AND EXITED THE VEHICLE WITH THE ENGINE RUNNING. UPON EXITING, VEHICLE BEGAN TRAVELING IN REVERSE AND TRAVELED BACKWARDS INTO THE GARAGE HITTING ANOTHER VEHICLE AND THE GARAGE WALL. HAVE OWNED THE VEHICLE FOR 8.5 MONTHS AND APPROX. 21,000 MILES AND IS VERY FAMILIAR WITH THE "ROTARY SHIFTER". HAVE NOTICED A FEW OTHER OCCASIONS THAT THE TRANSMISSION WAS PLACED IN PARK AND AN ONSCREEN WARNING CAME UP "VEHICLE NOT IN PARK". 17 DAYS PRIOR TO THIS ACCIDENT THE VEHICLE WAS PLACED IN PARK AND WHEN THE OWNER EXITED THE VEHICLE AND CAME TO THE REAR, HE NOTICED THE REVERSE LIGHTS WERE ON HOWEVER THE VEHICLE DID NOT MOVE DUE THE INCLINE IT WAS PARKED ON. I BELIEVE THE ROTARY SHIFTER FOR THE TRANSMISSION IS FAULTY.