WHILE TIGHTENING THE HARNESS STRAPS ON THE CHICCO NEXTFIT CAR SEAT, THE PLASTIC HARNESS TIGHTENING BUTTON POPPED OUT OF SOCKET, EXPOSING A METAL SPRING, WHICH STABBED MY HAND. THE PLASTIC HARNESS TIGHTENING BUTTON WILL NO LONGER STAY IN SOCKET, AND MAKES IT DIFFICULT TO LOOSEN AND TIGHTEN THE CAR SEAT FULLY. THE CAR SEAT WAS IN THE REAR FACING POSITION AT THE TIME OF THE INCIDENT.