I remember, back in the last century, a TV station did a test, where they went to a whole bunch of supermarkets, and brought stuff. Some was on sale, some was not.

What they found, was that every (100%) error in price went to the favor of the company. Not one single error was in favor of the customer.

Often, the stores were good about correcting the error, but the cashier could never do it. They always had to go to the service counter.