I recently heard on a podcast where one of the guests recounted what his father used to say about the employees making cash-handling mistakes in the small store he owned. It was something like, "if it was merely incompetence, you'd think half of the errors would be in my favor."

It probably is a glitch in this case, but it's hard not to see the dark patterns once you've learned about them.

If you short charge a customer they will demand correct change if you overpay a customer won't complain. In the cases of customers giving back extra money it becomes neutral.

His father's theory didn't take into account this.

Incompetence, filtered by customers biased to complain when cheated, and ignore mistakes in their favour?