I'm an American and I work at FastFoodRestaurant in Germany. I'm not sure if this is a cultural thing or a customer thing, but it never ceases to amaze me how indignant people are about being offered things they didn't order for free.
This is just the most recent example, but the other day, a husband and wife drove up to the drive thru and I was giving them their order (note: the people who take your order are not the ones who pack it. We just see the items on a screen and pack them accordingly). I hand them the last item, a large chocolate shake.
The man goes to take it and the woman informs me she ordered a small. I look back at the screen and see it indeed says large and tell them that it says large on the screen, so they paid for the large. The husband starts to say they'll just take it anyway, but the wife puts her foot down, crosses her arms and refuses to touch it. I take it back and tell them I'll give them the 30 cent difference back, because I happen to have a manager card right there to open my register with. I grab the change and hand it to the man and give him the large shake, which he proceeds to take again.
"NO," says the wife, "I ordered a small and I want a SMALL. Not a large, a SMALL."
I was just standing there pretty helplessly because I had already given her the money back. I tried to tell her in a friendly way that she can have the large for the price of the small since it was our mistake.
"I want a SMALL for the price of a SMALL," she screams while husband stares ahead with dead eyes. I ask her, "So you want me to throw this one out and make you a new one in a small container?"
So I pull the trash can right over in front of me so she can see, take off the lid and pour it out. I go over to the machine, fill a small container, and give it to her. "Was that so hard?" she asks me. They drive off into the sunset.
I'm not usually so theatrical about it, but it was pretty slow and no one was behind them. I was just trying to be nice and not waste more food. Fuck me, right? :P