Adding more fuel after the gas pump clicks off can damage the charcoal canister by saturating it with fuel and cause the check-engine light to illuminate. If the ORVR is damaged, it won’t adequately capture those harmful vapors. The system typically lasts the lifetime of a car, but topping off could lead to an avoidable repair that could cost hundreds of dollars.
Something else to keep in mind is that topping off your tank could cause gas to spill on the ground, which is a safety and environmental hazard.
The bottom line is that topping off your car is dangerous to you and your car, and may cause problems. Your best bet is to stop pumping when the fuel pump clicks off.
