Okay, let us imagine a small situation.
You buy something from a store, but right after you left, you change your mind and you want something else. You come back, place your plastic on the counter and explain to the cashier that you changed your mind and you want to trade what you have for another product of the same value, the cashier agrees to change it. The cashier picks up your bag and put the stuff you want inside, the cashier hands it back to you.
You look inside your bag, you see that you have the thing you got first plus the stuff you wanted. Do you warn the cashier that he might have committed a mistake or do you just walk away with both?