Consider this scenario:
You go to a store in hopes of buying a certain item. Prior to going to that place, you already know the usual price of the "ordinary" (read: regular) merchandise, so you have your purse filled with coins, but you bring extra, just in case you find a "special edition".
The store, thankfully, has the item available. You get excited. The thing is, their price is thrice the value of the regular edition (so it's the price of the "super special edition"). Even though you have doubts that the salesperson will give that super special edition, you haggle to lower the price to twice the regular price. The salesperson agrees.
You get the item. It was indeed the regular edition (or at least you feel this was the regular edition, based on your knowledge of the item). In the end, you secretly paid half of your agreement with the salesperson, that is, you paid for what you thought the price of the regular edition is to begin with. The salesperson was not aware of what you've done... at least not until later, when you're already out of the store.
Your conscience starts nagging you. On the one hand, you got the item, regardless of the edition. You had your doubts and you know the regular market price, so why didn't you haggle to that price in the first place? Because you feared the salesperson won't sell you the merchandise?
On the other hand, you knew the salesperson was overcharging. You really wanted the item. You just paid its actual worth. You're not going to that store anytime soon...
Would you feel guilty?
(Image by David Lowe, from "Six Stages of Grief" series)