Gill, the difference there is that you asked. That is not a good example. What I am talking about is if you had said nothing, knowing that they didn't charge you the fair price. You asked, you made sure, you didn't knowingly and willfully rip the pizza store off. I don't think that you did anything wrong. You would have been wrong, in my book, had you not asked. Had you taken advantage of a mistake and hoped that they didn't notice so you could save some money, then you are wrong.
Had the guy asked for 27, and hoped that you would pay 27 so he or the store could keep the extra 5, then that is equally as wrong. If he asked for 27, and then you asked "are you sure it isn't 22?" and then he realized his mistake, then charged you the 22, then it is just a mistake.
There is a significant difference between mistakes and willful taking advantage of a situation. You asked to make sure that you were paying what was correct, so you fulfilled your obligation to pay the fair price. You can't fix stupid. But, you can do the moral thing and make sure that you are paying the fair price.