Worst customer service I ever seen

The other day, I was on McDonald's drive through with my girlfriend, I only order a a drink, and she ordered crispy chicken sandwich. After we get by the window, the lady who handed the order to us was wrong after we open it. So, we go back and tell her about it. She seems unhappy, and mumbling with her mouth. I told her what is wrong, that is not my problem, you made that mistake, not mine. I just want that crispy chicken sandwich, and I have my receipt here, would you like to take a look at it? I meant it is no big deal if you made a mistake, I would forgive you. But don't give me that face, it seems like I try to frame her.