One shopping bill in six is wrong although the study of checkout errors showed that sometimes the mistakes can be in the customer's favour.
Volunteer shoppers at different stores bought the same 33 products in shops throughout Britain during a 12-week period in the survey for The Grocer magazine.
The mistakes included being charged twice for the same item and sometimes being billed for goods not even in the shopping basket. Hunting out discounted bargains was at times a waste of effort as the discounts did not register on the till and were sometimes replaced by more expensive prices.
In the North-east, one shopper found he had been charged twice for a pounds 1.25 bottle of Diet Coke at Sainsbury's, while at Pontypridd, Wales, another was charged for something she had not bought. Occasionally the shopper actually won in the checkout lottery - one customer discovered he had not been charged for a bottle of wine.
The errors occurred at Sainsbury's, Tesco, Safeway and Morrisons, despite the fact the industry had invested millions of pounds on new technology at checkouts, including barcode scanning. The supermarkets said most of the overcharging could be put down to human error.
Sainsbury's said shoppers sometimes picked up the wrong product or misread special offer information. Tesco said it always retrained staff if mistakes came to light.
Consumers could be said to be, in part, authors of their own misfortune. Few check their till rolls carefully after taking the shopping home, and even if they discover minor discrepancies they often do not bother to complain.
A spokeswoman for The Grocer said: "We found human error was to blame in most cases. The speed at which some checkout staff pass items over scanners is bound to lead to mistakes.
"Very few people would notice these mistakes. We are talking about pennies. But we have spotted it as an issue and our shoppers have spotted these mistakes on their till rolls - it is a concern."
A Consumers' Association spokeswoman said: "The only way to be absolutely sure you are not paying over the odds is to go to the supermarket equipped with a calculator, notebook and pen, taking down what everything should cost and keeping a running total which can be compared with the total charged at the till."Reuse content