The Frugal Duchess offers tips on how to catch errors when the items in your shopping cart are tallied up.

In the current environment of massive fraud and monetary mayhem, I recently encountered a small financial error. But I refused to swallow a 25-percent loss at the checkout counter, where I was charged an extra $1 for a pint of organic grape-sized tomatoes.

As a weekly special, the advertised price was $3.99 per container. But when I studied my receipt at the checkout lane, I noticed that I had been charged $4.99 for the tomatoes.

Here’s what that episode taught me:

Always check receipts. Checkout errors are common, according to one industry survey. In 2003, A.T. Kearney indicated that there are “data errors” on 30 percent of retail items. My shopping bags verify those findings, and after a spate of checkout errors at several stores, I’ve learned to quickly review receipts before leaving a store’s parking lot. Many errors are computer-driven, explained one store employee. Occasionally, sale prices and special markdowns for various items are publicized but not updated in the store’s computer system. For example, the scanning code on my tomatoes did not reflect the hand-written sale price spiked to the produce display.

Check the error policy. Some stores will compensate shoppers for mispriced items. In my neighborhood, Publix, a regional grocery chain, will give you an item for free if “the scanned price of an item … exceeds the shelf price or advertised price.”

Examine large purchases. “You have to really scrutinize receipts, especially for big-ticket items,” said Jane Bennett Clark, senior associate editor at Kiplinger’s Personal Finance. During a recent interview, Clark identified three tiers of pricing errors.

1. Customer errors occur when shoppers select an item that is not covered by a sales promotion. 2. Due to employee error or computer glitches, merchandise is mislabeled.

3. Bait-and-switch gimmicks occur when shoppers are lured into a store with the promise of low-priced specials, but are directed toward comparable merchandise at higher prices. The third scenario is against the law and shoppers should contact local regulators if a bait-and-switch scenario is suspected, Clark said.

Frugal Duchess is an occasional feature. E-mails can be sent to Sharon Harvey Rosenberg at sharonhr@bellsouth.net. Sorry, no personal replies.