Employees were troubleshooting dual duct detectors in a room. The injured employee was on the second rung from the top of a forward-facing 12-foot A-frame ladder. He was reinstalling the duct detector in the furthest duct from him (18 inches). When he pushed the duct detector into the existing holes, the duct broke free and fell, striking his back and shoulders. He was knocked forward over the ladder and fell to the floor. The employee sustained a head injury.