A new investigation from Consumer Reports, The Guardian, and the Food & Environment Reporting Workshop found customers who shop at Kroger have been unknowingly paying full price for discounted items.

The investigation alleges price tag discrepancies at the supermarket giant’s stores leading to customers being overcharged on many items marked as discounted or on sale.

“People should pay the price that is being advertised, that’s the law,” Edgar Dworsky, a consumer advocate and the founder of Consumer World, said in the investigation. “The issue here is that shoppers can’t rely on the shelf price being accurate, and that’s a big problem.”

According to the complaints the publications reviewed, while Kroger employees work quickly to correct pricing errors, many of the mistakes go unnoticed.

In a statement to the publications, a representative for Kroger said it is “committed to affordable and accurate pricing” and that it regularly conducts price checks that review “millions of items weekly to ensure our shelf prices are accurate.”

“While any error is unacceptable, the characterization of widespread pricing concerns is patently false,” the company said.

Read the full investigation here.