It's wrong to make people wait.
Carl is at the grocery store checking out when he realizes that he forgot to get the milk.
Carl wants to buy milk.
Carl checks out and brings his groceries to the car then returns into the store to buy the milk.
Carl tells the cashier to wait while he goes to get the gallon of milk.
Carl gets home a little later than planned, but with all the groceries.
When Carl gets back to the register, he sees that the cashier has had another worker return all his things to the shelf.