Once you get to the checkout, shopping stops. Anything short of the junk and magazines in the racks at the actual checkout lane is continued shopping, and you should forfeit your position. If you're waiting and you can get something and get back before you have to make others wait, that's fine too. But once you are about to pay for your stuff, you say to the checkout person "oh, just one minute, I forgot something" and you vacate the checkout, you've gone too far and deserve to have those bagels, milk, toilet paper, whatever stupid-ass-thing you forgot shoved into your ears, nose, eye sockets, and any other orafice suitable (or unsuitable for that matter) for item reception. I just re-e-EEEally appreciate the selfish cockups who think it's okay to make me and the others behind me wait in line while they continue to shop. If you forgot, make another run, be somewhat CLOSE to polite. This is most certainly a capital punishment offense, if only for example's sake.