When a coolant reservoir overflows, a bit of coolant liquid escapes the reservoir and the engine's temperature heats it. Because of this, the coolant emits high pressure which could damage the wiring over the engine and electrical components. One of the major consequences is engine overheating, which could lead to permanent damage done to the engine. The overflow hose can also be damaged and it could prevent the engine from receiving the needed coolant to hold the heat down.