For those who live in Canada
National Fire Code
Part 4.5.8.6 (1) Except as provided in Sentence (2), flammable liquids and combustible liquids shall not be dispensed into the fuel tank of a motor vehicle or watercraft while its engine is running
Part 4.5.8.6 (2) Diesel fuel may be dispensed into the fuel tank of a vehicle while the engine is running provided it is dispensed at least 7.5 m from gasoline dispensing units
I would bet that the American Fire code is identical but I don't have a copy of it at work (because I dont enforce it)
Not being there when you car is being filled is just not smart, not only is it dangerous, its a big legal liability because you would be responsible for the cleanup costs.... if that stuff gets into the sewer or on the dirt ground, you are talking major $$$ to clean up.
If I see anyone jamming something in the handle and getting back in the car, or leaving their car running, I press charges. If you spill and you did that, guess who's paying for a phase 2 environmental assessment?!? there's about $5000 for you to pay, enjoy it.....