Transcript for Supermarket Surprise: Are You Paying More Than You Should?
Next tonight, our ABC news investigation. The surprise at the supermarket. When your items are scanned at the checkout, are you paying more than you think, and more than you should be? ABC's Mara schiavocampo tonight with three things you should do when you check out. Reporter: It's the supermarket error. That may be eating into your grocery budget. We did over 1,300 investigations of supermarkets. And over the half the time, we found violations. Reporter: ABC news followed an undercover inspector from new York City's department of consumer affairs on two separate Dames of unannounced grocery store visits. The inspector choosing items at random, then identifying himself as checkout. I actually work for the city. Reporter: Of the five stores we visited, four received violations. In one case, scanner mistakes adding a whopping $13.08 to the $114 bill. An increase of more than 10%. Take this olive oil, priced at $7.39 on the shelf -- It's ten cents overcharged. Reporter: Ringing up at $4.49. Common vie laugss, charging for the contain herb at the deli counter which is supposed to be free. And charging tax and items that should be tax exempt. Pain medication and most foods. Someone might say, it's just ten cents, it's not a big deal. Is it a big deal? Absolutely. Five cents, 20 cents, dollar here, that's happening week in and week out. Reporter: Officials say the best protection? Review your receipt item by item and if you are wrongly rung up, ask the store for a refound of the overcharged amount and report them to a consumer protection agency. Tips for stocking the fridge without draining your wallet. Mara key Campeau, ABC news, new York.
This transcript has been automatically generated and may not be 100% accurate.