1955 doubled-die cent
When a modern coin die is created, it is struck from a working hub, which places the incuse image onto the die that will subsequently be used to strike coins. Normally, this requires multiple blows. In 1955, one of the working obverse dies at the Philadelphia Mint was misaligned on the second blow from the working hub, thus resulting in a doubled image. Due to the manner in which this hubbing was carried out, it most noticeably affected the date and inscriptions, with very little doubling (albeit noticeable loss of detail) visible on the bust of Lincoln. These doubled features were visible on all of the coins struck from this die. It is estimated that 24,000 of these coins were struck, all during one night shift at the Philadelphia Mint.
They were originally found in New England, and many were distributed in cigarette packs in vending machines. The price of the cigarettes was 23 cents per pack, and two pennies were included to even up the price to the quarter required to buy from the machine.
The 1955 doubled die is one of the most famous die varieties in US coinage. Very few exist today in totally mint condition, as all were discovered while in circulation. Over the years, many counterfeits of this coin have surfaced. It is advised for collectors to seek expert opinion before auctioning for these coins if not certified by one of the top numismatic certification companies.
A seemingly similar variety to the 1955 doubled die is the so-called 1955 "Poor Man's Doubled Die" cent, created by die deterioration doubling. It is caused when the design on a worn die becomes eroded and distorted, causing part of the design (such as the final digit of the date) to appear doubled. It is much more common than the actual doubled die, and as such it sells for only a few dollars.
A 1955 doubled die Denver mint penny was a plot device in the American movie UHF when a rich man cruelly gives a penny to a beggar. The penny is valuable and the beggar ends up with wealth after the rich man's avarice has bankrupted him. The actual 1955 doubled die pennies were from the Philadelphia mint, not Denver.