PARKED A STOP LIGHT WITH ONE VAN AHEAD OF ME SEAT BELT THAT WAS HOOKED ON THE MECHANISM THAT CONTROLS THE BACK OF THE DRIVERS SEAT SLIPPED OFF AND THREW THE SEAT BACKWARDS, FORCING ME TO TAKE FOOT OFF BRAKE AND HIT THE CAR AHEAD OF ME AND CAUSING ME ( BY THE FORCE I WAS THROWN BACK) SEVERE HEADACHES. NO CORRECTIVE ACTION TAKEN. *NM