Sept. 13, 2010 -- Each year, consumers lose $1 billion to $2.5 billion dollars because of scanner errors.
Even though sometimes mistakes at the checkout counter come out in the customer's favor, that doesn't help the customer who was overcharged.
More errors are found during times of high-volume sales – like holidays, but it can also happen on everyday purchases, as we found out.
We're tagging along – undercover – with inspectors from New York City's Department of Consumer Affairs. Their mission is clear: make sure customers are charged correctly.
CLICK HERE find out how to report mistakes, state by state.
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First, they select random groceries to make sure the price listed on the package or shelf matches the price at checkout.
The most common problems are sales not programmed into the register, scales that charge for the plastic package when they're just supposed to charge for the food, and stores that charge tax on non-taxable items.
"It shouldn't be a consumer's job to have to bird-dog checkout," Jonathan Mintz, New York City's consumer affairs commissioner, told "Good Morning America."
Mintz's New York City Consumer Affairs Department recently conducted a sweep of nearly a thousand supermarkets. More than half failed inspection.
"I think the real question is not are they trying to purposely cheat customers, but are they really making the effort to make sure that their customers are charged appropriately," Mintz added.
New York inspectors recently found 16 violations – like a bottle deposit charged when it's not required. And no price per pound listed on deli salads.
"If they're charging it by the pound, then they should tell you how much it does it cost by the pound," Mintz, New York City's consumer affairs commissioner, said.
The sticker price on a package of salami was $3.09. The register price was $3.49 – a 40 cent overcharge in the store's favor.
"It really adds up. It adds up for people who are really watching their budget," Mintz said.
It's not just New York. In North Carolina, state testing found 5 percent of products rang up wrong. In Wisconsin, 4 percent were high and in California, 3 percent. Vermont cited a quarter of its stores for scanner violations. And Arizona inspectors found 91 scanner overcharges earlier this year worth a total of more than $100 dollars.
In California, authorities took the unusual step of filing 62 criminal charges against Ralphs Grocery chain. One allegation was that the stores charged for the weight of ice coating seafood.
"9.99 a pound shrimp is being sold with ice on it so the customer is actually paying 9.99 for some water," said Deputy Los Angeles City Attorney Don Cocek.
Shopper Banned from Two Supermarket Chains
"We take our relationship with our customers very, very seriously and … we will do whatever it takes to make this right," Kendra Doyel, Group Vice President, Marketing, Ralphs Grocery Company, said.
Ralph's says that it took care of the majority of the violations when they were issued and that it continues to work with the city to resolve the charges.
So is it worth it to spend your time and energy searching out overcharges of a few cents? Alana Lipkin of Framingham, Mass., thinks so. She's spent more than a decade taking advantage of store policies that give you groceries free if they ring up wrong.
"In a single shopping trip the largest dollar amount was over $1000," she said.
Eventually, she was banned from two New England supermarket chains. They say she was taking advantage, but she says stores are.
"It may only be 2 percent but when you think of how much money Americans spend on a lot of items that's a lot of money," she said.
The Food Marketing Institute, which represents grocery chains, told "GMA" that supermarkets strive for 100 percent price accuracy even though an average store stocks about 45,000 different products, and that in a recent national study, supermarkets had the best accuracy rate of any industry, but that shoppers should immediately report discrepancies.
You should either jot down shelf prices or snap a cell phone picture of them. If you don't want to do every item, then choose a few – maybe 10 or 12 – and spot check.
If you can't keep up at checkout, review your receipt before you leave the store parking lot so you don't have to schlep back for a few cents' correction.
And for more motivation, find out which stores offer you items free if they ring up wrong.
If you find a pattern of overcharges, as opposed to a few innocent mistakes, complain to your city, county or state weights and measures department.
If you want to complain about overcharging at a grocery store – or any type of store – contact the state department that oversees the issue. Click HERE for a list of those agencies, listed by state.