It's worth knowing that these machines aren't infallible."

"I think it should be of real concern to consumers. If you translate it into monetary terms, it has to be in the millions of dollars. "The implications from a study like this are massive. Consumers in New Zealand are not making enough noise about it yet," he said. In one case, a supermarket offered bottles of coke for 99c, but displayed the wrong size bottles under the sign, and charged the shopper $3.

In another, a cooked chicken was advertised on sale for $6.99, but the original price of $11.99 was charged at the checkout. In this case, Pickering said, the mistake was spotted and the customer was given the chicken for free. But more often, retailers respond badly when errors are pointed out. Shoppers who took part in the study reported sales people "rolling their eyes" when asked for a refund, having to fill out forms to get a $2 overcharge returned, and being offered cash refunds only when the original purchase was by credit card. "The errors are happening right across the retail area, particularly large-scale stores," Pickering said. "When you're pumping 100 items through a till, it's much easier to make a mistake and much more difficult for the customer to keep up with the things flying through the scanner."