In 1943, due to a shortage of copper during World War II, the U.S. Mint produced pennies from zinc-coated steel.
This is a 1955 Lincoln cent where the obverse (front) die was hubbed (stamped) twice, creating a distinct doubling effect on the lettering and date.
Some 1970-S Lincoln cents have a smaller date than the normal version. These are quite valuable, especially in mint condition.
In 1982, the U.S. Mint did not place mint marks on dimes produced in Philadelphia.
Some Wisconsin state quarters from 2004 exhibit extra leaves on the ear of corn on the reverse side.