Jerusalem Artichoke.

The Jerusalem artichoke is neither from Jerusalem nor an artichoke; it belongs to the sunflower family. The Italians called it girasole articciocco, "sunflower artichoke". Over the years girasole became "Jerusalem". (source)

There is no silver in German silver. It consists of copper, zinc, and nickel. As well, it was first produced in China, not Germany. (source)

White chocolate contains no chocolate. It is made with equal parts vegetable fat, milk solids, and sugar, along with a small amount of vanilla. (source)

A ten-gallon hat actually holds less than a gallon. (source)

A peanut is not a nut. It is a member of the pea family. (source)

The Hundred Years' War actually lasted 116 years, from 1337 to 1453. (source)

The English horn is not English. The instrument was brought from the Near East to Europe by returning Crusaders. The French called it cor anglais, not because it was English, but because it was angled to enable a player to reach the lower keys. As well, it is not a horn, being a woodwind instrument.

The "koala bear" is a marsupial, not a bear.

Lead pencils contain no lead; this is a misnomer. While red lead is an oxide of lead, and white lead is a carbonate of lead, the "black lead" used in pencils is neither a metal nor a metallic compound. It is plumbago or graphite, one of the forms of carbon. (source)

The Byzantines never called themselves "Byzantines". This term is derived from Byzantium, the Roman name for Byzantion, the original name of the city that is now Istanbul. It was first used in 1557, over 100 years after the fall of the "Byzantine" Empire, when German historian Hieronymus Wolf published Corpus Historiæ Byzantinæ. The "Byzantines" called themselves Romans, while they were called Greeks by Western Europeans. (source)

Arabic numerals are not Arabic. While Europe obtained this system from the Arabs, the Arabs in turn obtained this system from the Hindus around the middle of the eighth century. The Hindu writer Aryabhata first described the new system in the year 499. The invention of the sign for zero made arithmetic computation much easier. In contrast, calculation was more awkward in the Roman numeral system. (source)

Generally, centipedes do not have 100 legs, nor do millipedes have 1,000 legs. Different species of centipedes have between 30 and 346 legs. Different species of millipedes have between 40 and 750 legs. (source)