Have you ever noticed that the nozzle for diesel fuel simply will not fit into the gas flap of a car designed to run on regular gasoline? I'm not sure if this is universally true, but I'm glad it was true at the gas station where I stopped and tried to get gas while thinking about about a million other things today. I'm pretty good at keeping diesel and gasoline straight when they're at separate pumps, but when they're coming out of the same machine, golly, anyone could make that mistake.