ResearchA Taxonomy of Coin Mint Errors
The United States Mint is a factory-type assembly line, printing the money and minting the coins we use daily. And like any factory environment, they can make mistakes. You may call them freaks, bloopers, or errors - the official word is "misstruck coins". They aren't always worth very much, but some collectors treasure these over all others, while others turn up their nose at them. The novelty value is not to be denied; coins that are half-there, rotated, or impressed with different denominations on front and back definitely ring in as something you don't see every day.
Clipped Planchet - the mint starts with these smooth, blank disks, which are the planchets. The planchets ride a conveyor belt and the dies machine-stamp the coins in what is called a "strike", making the design on the coin - there goes Lincoln's face on another penny. In the case of a clipped planchet, the planchet was cut from an area of the sheet metal that already had another cut from it - the result is a half-moon shape, which then may still receive the die strike and make it out the door.
Multiple Strike - whoops, we hit it twice! These are particularly striking when neither strike was on center, producing only a partial design on either side, like a film reel strip halfway out of line with the frame.
Blanks - never struck at all. Will be the same metal composition and size as the finished coin, but with no strike and hence smooth.
Defective Die - the die used to strike the coin itself was faulty. Cracked dies, dies with pieces missing, or dies so badly worn they barely made an impression can all make curious differences in the coin's appearance.
Off-Center Strike - whoops, we missed! Only part of the coins surface design met the planchet here, resulting in a partial design banked by a half-moon of smooth planchet.
Broadstrike - In this case, the planchet was riding slightly out of it's collar, and the force of the strike produced a slightly jittered design and the edge of the coin got curved or spread out. Usually produces a bowl-like or bottle-cap shape.
Lamination Error - This is a coin with part of it's surface missing. Back when the planchets were cut, there might have been dirt or gas oxides in the metal alloy. So the metal of the coin appears to be flaking off like old paint.
Brockage - A coin previously struck stuck to the die and hit the next coin, producing a mirror image of it's design on the next coin! The result will have the same design on both sides, but one side is mirrored from the facing side of the previous coin.
Wrong Planchet - Whoops, we made a quarter-sized nickel! Or a dime design on a penny. Pretty self-explanatory.
Rotated Die - US coins are struck with the two sides' tops rotated 180 degrees from each other; however, sometimes the dies rotate so the two sides are at 45 degrees or even at zero, so the two sides align like a medallion.
Mules - The rarest of them all. Two different denominations of coin struck on the same piece - so it's a dime in front and a nickel in back, for instance.
The values are a potshot. The various kinds of errors may raise a coin's value from a few cents to several hundred dollars, with a multiplied value for higher-denomination coin. Having the date and mint mark visible also adds to the value. If you find an error coin, you should take it to several different dealers for appraisal. One dealer may not be interested, but another dealer may have a regular customer who is enthusiast about collecting error coins.