I actually did that once. I was on my old '83 Shadow 750 on a long trip. I was tired & pulled up to a little country gas station. I took the fuel cap off, grabbed the closest nozzle & started pumping. An old guy came out of the garage area & asked me, "that thing a diesel?" Well, I was already tired, but I bit my tongue & said, "No, why do you ask?" He said, "That's what you're pumpin' there!" I hadn't noticed the larger diameter nozzle. I said a few expletives & hung the nozzle back up. I rolled it over to to side, the guy let me borrow a few tools & I proceded to drain the tank. Thank God I hadn't started it yet or I'd have had real problems. I filled it up again with premium, making sure this time to grab the correct nozzle. It spit, sputtered & smoked a little, but I ran it like a scalded dog the rest of the way home to clear the system out & it was OK.