Depends on how far you push it.
More often than not, the nozzle will shut off the flow of gas a fair amount too soon. Leaving the nozzle firmly in there and starting the flow again once can give you a little bit of extra gas without issue.
Pull the nozzle out a little, and/or keep pumping it in and at that point you're risking problems. Spilling gas out of the tank is never good, and a fuel tank that is too full risks damage to the vapor collection system.