Price difference in shopping bill and price tag

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When I was waiting for checking out in the shopping mall yesterday, a guy in front of me argued with a shop assistant for the matter of price difference in tag and bill. It's really a cheap price on the tag. However, the assistant insisted to charging him the amount as per the bill, but he is not willing to pay the higher part on it. Finally, the guy is allowed to take the goods with the cheap price. Have you had such unpleasant experience? What will you do?