Sometime in mid-July, the machines were given a "routine update" via a blast from a Walmart server, said Walmart spokesperson Ashley Hardie.
"A recent update caused a display error. It did not impact the price charged to customers," she said. "This issue has now been corrected, when we provided another update."
Update: The day after Walmart said the matter was resolved, a report out of Michigan late on Wednesday (Aug. 1) found Walmart self-checkout systems to be down entirely while waiting for upgrades and "that it was affecting other Walmart stores across the country." We are waiting to hear back from Walmart.
The nature of self-checkout makes this a relatively non-interruptive error; the receipt is the more important document. When customers do look at the screen as items are scanned, they are almost always checking to see if the item's price is correct, as opposed to watching to see if the total grows by the correct amount. That said, for customers who did notice the strange screen readings, it probably didn't make them confident the amount charged would be correct.
This is a potentially meaningful psychological issue with self-checkout. Even though the reliance on POS accuracy is just about the same with self-checkout as it is with an associate performing cashier duties (rare is the Walmart associate who will give any change amount other than what the POS tells them to, even if simple math makes it clearly wrong), the shopper's perception of reliance on the system is much higher with self-checkout. Therefore, it's much more of a problem to have this type of glitch in a self-checkout lane than a staffed lane, presuming the store's objective is to move as many customers through lower cost self-checkout lanes as possible.
This incident is just what self-checkout needs right now. And it's also a great argument for shoppers using their mobile devices to see real-time charges on their payment cards or bank debit cards.