The Fuel Evaporation Control System in your vehicle is designed to prevent the release of fuel vapor into the atmosphere. It consists of a charcoal EVAP canister; purge control valve and purge vent valve, canister purge hoses, fuel tank vent lines, and a sealed fuel tank cap. The charcoal EVAP canister is used to store fuel vapors until the engine in started. When the engine starts it creates a vacuum that draws in the fuel vapors and burns them instead of releasing them to the atmosphere.
Overfilling your gas tank can lead to liquid fuel entering the carbon canister and damaging it along with the purge control valve. It may also feed fuel directly to the intake system which will enrich the fuel mixture and foul the spark plugs. This will turn on your check engine light and probably result in a repair bill of several hundred dollars. So the moral of the story is… Do not top off your gas tank, unless you enjoy supporting your local auto repair shop.