It is illegal to serve margarine in a restaurant without explicit warning.

The Law

In the late 19th and early 20th centuries, the dairy industry fought fiercely against the rising popularity of cheap butter substitutes. To protect local dairy farmers, Delaware passed a law requiring any public eating establishment serving oleomargarine to post highly visible signs declaring it. Sneaking yellow-dyed vegetable oil onto a patron's toast was considered the ultimate culinary deception.