SOME decades ago, a grocery store’s aisles were often filled with “chunk-a-chunk-a” sounds, as clerks stamped prices to the tops of cans and boxes before putting them on shelves. It was a labor-intensive operation, but it did result in a price being affixed to most every item in the store.

Then bar codes and computerized cash registers arrived. In most stores, prices were posted on shelves but not on the items themselves.

I’ve always trusted that the system works well — and I’ve tapped my foot impatiently when a shopper ahead of me slowed the checkout process by closely watching the prices that came up, as if the scanner might have recorded the wrong product code. What I hadn’t realized was that there is valid reason to be vigilant. The potential problems originate on the shelves, in the form of the shelf tags, which may or may not match the current price in a store’s computer.

A typical grocery store puts 5,000 items on sale in a week and removes sale prices from another 5,000. That creates an abundance of opportunities for mismatches when workers print out the new price labels in a back room, then hunt for the proper place on the shelf to attach them.