The fixed contact in the starter solenoid switch can wear prematurely as the engine starting operation is repeated, because heat from the engine increases the temperature around the starter motor, and because the shape of the fixed contact is improper. The moving contact can be caught with the worn and unleveled fixed contact causing excessive stress, and the insulation bush is eventually damaged causing a short circuit, in which case the starter motor keeps running, and could become overheated and start a fire, which in the worst case could spread inside the engine compartment.
