driving over an unexpected rail road track. the car jumped and i lost my AC belt. i don't know if it was debris or if it was the belt itself (doubt this).
i have also lost my timing belt. not because of debris but because of a broken idler pulley. and considering the amount of damage done to the timing belt and the valves and the amount of work required to repair it (especially after a long rebuild), i would do ANYTHING to make the timing belt last. that includes keeping the covers on to prevent any type of debris from falling in. there's a reason why the service manual mentions to keep the pulleys and cam gears clean and dry.