Technical issues are being blamed after reports surfaced that Walmart customers in mid-March were overcharged at thousands of stores across the U.S.

The incident, first reported by Bloomberg, involved self-checkout kiosks at 1,600 Walmart locations. The issues, including customers being overcharged while others paid too little, began March 19 and continued for several days affecting prices for items such as food, clothing and appliances. Traditional checkout lanes and online orders weren’t affected.

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Walmart confirmed the issue in a statement to Reuters.

“We’ve made it a priority to refund customers who were overcharged, and we did not take action on the undercharges with our customers,”  Walmart spokesperson Mischa Dunton said.

Walmart, the nation’s largest retailer, did not confirm how many shoppers or which stores were impacted. The company has some 5,000 locations in the U.S.

According to the reports, the issue was due to an internal system failure that stopped price data from flowing to the self-checkout kiosks. The issue was eventually resolved and more than 80% of overcharged customers were refunded.

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