Mastermind
Growing up I worked at a chain grocery store. They had a policy in place that if an item price label didn’t match the scanned price in the computer, the first item was free and any subsequent items would be for whichever price was lower.
Every few months an elderly woman would come in and spend 4-5 hours shopping. When she would come up to the register, she would have a full cart and near everything would be free. I had to ring her out a handful of times and only caught a few items overall that she was wrong on.
At one point, she found a comforter BBB mislabeled and got it for free ($40-50 sticker price). More than once the store manager would watch her on the store cameras to ensure she wasn’t switching the stickers but never saw her do anything sneaky, just very calmly looking over every item shelved one at a time.
Eventually the policy changed and she stopped coming in at all.
Story credit: Reddit / DrunkPhoenix26