The contact owns a 2004 Ford F-150. The contact stated that while parked when the vehicle suddenly rolled backwards downhill the contact's son attempted to get into the vehicle to make it stop. The trailer's jackknife stopped the vehicle from going onto the road. The contact manually moved the vehicle back into their garage. The contact stated their son did receive injuries to their head, left or right leg (the contact was unsure of the exact leg), and their left or right arm (the contact was unsure of the exact arm). Medical attention was needed. The dealer was not contacted. The manufacturer was informed of the failure and opened a case. The manufacturer referred the contact to the NHTSA Hotline to file a safety complaint. The failure mileage was 74,000.