I remember a BAC 1-11 that had steering problems after landing, had to be towed off the runway. Inspections were performed, no fault found, off you go, on return the same thing happened again, more inspections, functions, etc, no fault found. The third time it happened, some bright fitter decided to have a look around before they towed it off the runway. He found a water leak from the forward toilet had dripped on to the nose wheel steering control cables and frozen solid. The reason it hadn't been found before is that by the time the aircraft had been dragged off the runway and inspections began it had all melted away.