Milk Shakes at just about any fast food restaurants. The machines always seem to be down! Pulled up to an Arby's a few months back to order a Jamoca shake and they said their machine was down. I asked them if I had pulled into the McDonald's drive through by mistake. We all had a big laugh... but that was months ago and they have now put a sticker on the menu that says their shakes are not currently available :(
Went to a Baskin Robins to order a Chocolate Milk shake and they were out of Chocolate ice cream... on multiple occasions. At least they don't have a machine to break down!