The item rings up for a lower price, what do you do?

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It happened many times, especially at the grocery store, that either the price for the item I am trying to buy comes up much lower than what was on the shelf or, when I am purchasing say 10 boxes of something, the cash register person charges me for less boxes. What do you do? Do you say anything? I usually do say something, and once the lady said to me: "What are you, a boyscout?" What about you? Do you let it go or do you say something? Is it theft? Or is it the problem of the attendant of the cash register?