My worst wasn't a part so much as the entire car. Towing a rolling shell to a customer who had cash waiting the thing caught fire. No motor, no tranny, no fuel and no battery and this thing burnt to an unrecognizable crisp in fifteen minutes. So quick I almost didn't get it unhooked from the truck in time to save the truck. How you might ask? It started with a dragging brake caliper I didn't notice.