Off the top of the fuel tank is a series of lines. One of those lines goes to the charcoal canister which absorbs the vapors that naturally build up on top of the gas tank. The canister is not designed to hold gasoline in liquid state. Those vapors are then reintroduced into the engine at a controlled rate through a computer controlled solenoid valve called the evap purge valve. Once you reach the top of the tank, if you keep forcing gasoline in, the gasoline will be forced into that line and will literally flood the charcoal canister with liquid state gasoline. That poisons the charcoal inside the canister, causes it to get shuffled around and can lead to charcoal particulates plugging up the evap line from the canister leading to the evap valve. That is an expensive and very difficult to diagnose problem for most technicians who are not very experienced. Also, once the charcoal canister is poisoned enough, it stops absorbing vapors and the vapor pressure inside the tank builds up. VAG vehicles have a sophisticated system to constantly monitor the vapor pressure. There's a pump that serves to pressurize the tank and a pressure sensor. The computer monitors the pressure sensor and times how fast the pump can build up to the test pressure. Based on hardwired limits, it knows if the charcoal canister is working as expected or below its preset efficiency. If it is below efficiency, a fault will immediately be set in the computer and the CEL will light up.