Yesterday I was talking with a woman who works at our bank. She showed me a receipt from the previous day and said this mistake occurred because she hadn't been wearing her glasses when using the debit card machine.
She said it was because she had forgotten her glasses. And indeed she probably had forgotten to wear her glasses. But also, the mistake occurred at a popular downtown pub. Not something I'd flash around if I were a banker, and certainly not something I'd grant a customer permission to take a picture of.
At any rate, she was delighted the transaction was not approved.