Getting All Mustard Flustered

(Two customers order cheeseburgers which are identical, except one has ketchup and one has mustard. A while after they get their food, one of them comes back up, waving a burger and looking furious.)

Customer: “You gave me the wrong burger!”

Manager: “Oh, I’m sorry about that. What was wrong with it?”

Customer: “This has mustard! I didn’t want mustard; I wanted ketchup!”

Manager: “Okay, we can remake that for you. Did we switch the burgers around?”

Customer: *looking confused* “I don’t know. My friend grabbed my burger, and started eating it. She ate the whole thing before she noticed it had ketchup. So I need this one remade, since I don’t like mustard!”