@VDCdriver, I accidentally overfilled my tank a couple months ago. I was using up some stored gas before it got too old to use, and I actually did worse than what would happen during a normal top-off. Although I did have some difficulty filling the tank once after that, the charcoal canister eventually dried out, and everything is back to normal (thank goodness!).
It's the people who frequently top-off their tanks who will likely spend hundreds of dollars for a repair. I seriously doubt doing it once is that likely to do permanent damage.
Don't get me wrong, I am not defending the practice of topping-off. It's stupid and sometimes costly, but I see people still doing it all the time. The pumps seem to be so sensitive these days that they save many idiots from themselves.