20 cent coin
The twenty cent coin had one of the shortest mintages and lowest circulations in U.S. coin history. Its mintage for general circulation ceased only two years after it began. It was only released for circulation in 1875 and 1876. The coin was not reeded -- meaning it lacked vertical grooves on its edge -- and, despite its plain edge, was too easily confused with the quarter.
The 1943 steel cent, also known as a steel war penny or steelie, was a variety of the U.S. one-cent coin which was wrought in steel because of wartime shortages of copper (which was needed for ammunition). The steel cent is the only regular-issue United States coin that can be picked up with a magnet.
Flowing hair dollar
The Flowing Hair dollar was the first dollar coin issued by the United States federal government. The coin was minted in 1794 and 1795. In May 2005, a specimen struck during the 1794 production was sold in a private sale for $7.85 million, more than any other coin in history.
These double eagle gold coins had a $20 face value. Though 445,500 specimens were minted in 1933, none ever officially circulated and nearly all were melted down because of the discontinuation of the domestic gold standard that year. Now, these coins are extremely rare and valued at over $7 million.
Double strike coins
Every so often, the U.S. Mint makes an error when minting a coin. This drummer boy quarter has what's called a "double strike error." Such mistakes often increase the coin's value.
Three dollar gold piece
Only one specimen of the 1870 $3 gold coin was believed to have been struck, and it was sealed within the cornerstone of the new San Francisco Mint building. However, a second example turned up 37 years later. Today, the mystery coin is valued at $2.5 million.
Liberty Head nickel
The 1913 Liberty Head nickel is an American five-cent piece which was produced in extremely limited quantities without the authority of the U.S. Mint, making it one of the best-known and most coveted rarities in American numismatics. The coins were struck, without permission, by a U.S. Mint employee named Samuel Brown. Today, only five are known to exist: two in museums and three in private collections. All are worth millions of dollars.
1804 silver dollar
The 1804 Silver Dollar is considered to be one of the rarest and most famous coins in the world, because of its unique history. Silver dollars dated 1804 actually didn't appear until 1834, when the U.S. State Department decided to create sets of coins to present as gifts to certain rulers in Asia in exchange for trade concessions. Because the silver dollar was still in use but had last been produced in 1804, Mint employees struck several dollars with an 1804 date. Additionally, an employee of the mint in Philadelphia illegally struck a few dozen more examples between 1858 and 1860 using the old 1804 die, most of which were confiscated and destroyed, but a few of which survived. Around 15 specimens exist today, and they're worth as much as $47 million each.