Have you ever been at a cash register and just know you were undercharged or not charged at all for something? What did you do?

I was at a diner recently and asked the hostess to add a tip to the total. When I got the receipt, I had only been charged for the tip. I said to the hostess that I thought she made a mistake. When she realized that the mistake was in my favor, she was shocked that I said anything. I mean really shocked. It got me wondering just how many people were honest about these things. My gut told me most people, but her reaction told me otherwise.

I really think that most people who are aware that a mistake was made would let the cashier or hostess know, but could it be that I'm wrong. (My wife will tell you that it happens once in a while). So take the poll and let us know...and please don't lie about being honest!