In 1943, the U.S. Mint replaced copper pennies with zinc-coated steel cents because copper was needed for the World War II effort.

But a small number of copper-alloy blanks were apparently left inside the coin presses and accidentally struck with the 1943 date. The Mint has reported that approximately 40 genuine examples are known—making the 1943 copper cent one of America’s most famous error coins.

Collectors should be cautious: ordinary 1943 steel cents are sometimes copper-plated, and other Lincoln cents have had their dates altered to resemble the valuable rarity.

Sometimes one tiny minting mistake creates a legendary collectible—discover authentic currency, coins and more at ThePatriotVault.com.