If you overfill your gas tank you might notice a gasoline smell because when you overfill your gas tank it will flow to the charcoal canister where the fuel vapor is absorbed.
Having an overfilled gas tank may cause your engine to have poor performance and could even damage your engine if done too often.
I've overfilled my gas tank before and nothing happened though but I only did it one time and I drove the car around until the gasoline level in the gas tank went down to what it should be.