Also, yesterday I bought two greeting cards. I thought the total seemed high, but didn't want to be that person scrutinizing the receipt in front of the cashier, so I checked when I left and realized she rang me up for the same card twice.
I turned right around, told the cashier very nicely, she apologized and told me she couldn't do a return and got the manager, then went-off to help somebody else.
On my way out, I passed the cashier who had originally rung me up and made the mistake, and called out thanks to her. She said goodbye and sorry again for the mistake.
Everybody was happy!
And I couldn't help but think, "THIS IS HOW YOU ACT WHEN SOMEBODY MAKES AN INNOCENT MISTAKE!"