My son is a cook at west coast burger chain and says that they'll make stuff that customers order, even if it's no longer on the menu. For example, a burger is introduced as a limited time special, but someone comes in after the promotion is over.

While he occasionally grumbles about it, he said that they won't tell the customers no and will make whatever it is so long as they have the ingredients in the kitchen.

I've not yet tested it myself though My favorite burger is still on the menu.