2 Answers

Most cars have an overflow jug.You should be okay.Any excess will flow into the jug.

It depends how much you overfill it by.In a modern cooling system you dont fill it at the radiator like in the old days (there are a few exceptions like some Nissans) but at a clear plastic tank known as the expansion tank.When the engine heats up and so too the coolant it creates pressure inside the system but it needs some air in the system for that.When you top up at the expansion tank,which will be the highest poimdt in the system you will see a maximum mark usually at 3/4 its full volume leaving a 1/4 free to be full of air. As i said as the coolant and air heats up,the air expands and as its a sealed system it pressurises the system.Coolant that is pressurised has a higher boiling point which has obvious advantages.Now if you overfill the tank you will leave less space for air to pressurise,if you fill it right up theres nowhere for hot coolant to overflow so it'll try n come thru the gaskets,,water pump or thermostat