Good one...
Another supermarket checkout joke I read about was one in a British magazine, where they sell alcohol in regular supermarkets. It said that when you have a party organised and have to go and buy booze, buy the booze you want, and make sure you have the exact amount of cash for what you are buying. Now, chuck a packet of disposable nappies and a pack of baby food on top of the booze, and go to the checkouts. Wait until it's busy so there are plenty of people around, and in line behind you. When you get everything scanned through, and the girl tells you the total, say loudly "Oh, only have X amount of cash and haven't got my card...I'll have to put something back, no problem", and hand her the nappies and baby food and keep all the booze...just watch the horrified stares and whispers...