It is unlawful to serve margarine in a restaurant without a prominent warning sign.

The Law Explained

You cannot sneak cheap butter-substitutes onto a diner's toast without explicitly declaring it. Because Idaho has a robust dairy industry, agricultural lobbyists pushed hard in the mid-20th century to ensure restaurants posted signs reading 'Oleomargarine Served Here.' This was intended to protect local butter producers and prevent patrons from being deceived by yellow-dyed vegetable oil.