Overfilling your gas tank can lead to several potential issues. Firstly, it can cause fuel to overflow, spilling onto the ground. This not only wastes fuel but also poses environmental hazards and fire risks. Additionally, excess fuel can enter the evaporative emissions system, potentially causing damage to components such as the charcoal canister or vapor lines. Overfilling may also trigger the check engine light due to the disruption of the fuel system's normal operation. Moreover, excess fuel can saturate the vehicle's carbon canister, leading to poor engine performance or difficulty starting the vehicle. To avoid these issues, it's best to stop refueling once the nozzle clicks off, indicating that the tank is full.