The factory installed trailer hitch receiver fell off while towing an empty trailer. I installed the trailer hitch receiver making sure the knob was in the green markings. The trailer was empty and within limits for the vehicle. I towed it about a mile in town. When driving over a manhole cover at an intersection I heard a loud bang and the receiver fell off. The trailer safety chains continued to pull the trailer. When stopping the vehicle, the trailer hit the rear of the vehicle and caused damage. It could have been a lot worse had I been traveling at speed.