The most common type of error is strike doubling, also known as machine or mechanical doubling. This is when the coin planchet shifts just a little bit during the strike, which may be caused by minor vibrations within an old or maladjusted press. When this occurs some elements of the coin design appear to have a shadow or doubling. However, the doubled design is neither complete nor very strong.
A re-punched coin occurs when the die strikes the planchet twice. This created stronger and more widespread doubling on the coin.
Another type of error is when grease accumulates in the die and results in a mushy weakly-struck coin. Sometimes there may be very weak or even missing design elements, known as a grease-filled error. Very little premium on these coins.
Coins occasionally are struck when the coin slips in the collar. In this case there is more margin at 3 O'clock rather than 9 O'clock, for example. Sometimes the planchet slips outside the collar so much that the coin is only partially struck, and rotated, and the rest of the planchet is off center. This is known as an off-center error.
For example, here is a coin about 10% off-center:
And here is a coin about 90% off-center:
A minor broadstrike is when the the margin is greater to one side and the coin lacks reeding on the edge; in this case the coin diameter will be just slightly larger than typical.
When the coin has reeding and
typical diameter for the series, then the coin is known as a minor
misaligned die error (with very little premium
Another common error is a clipped planchet. Here is one example:
Sometimes when the die becomes very worn a piece will eventually break away. In this case the metal will flow into the hole on the die and the result on the coin is a blob called a cud error which typically forms on the coin periphery.
A planchet error is similar, resulting from a broken die, but is usually not where a piece broke off the side as a cud. See the planchet error on this Buffalo Nickel?:
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TYPES of DOUBLING
Telling the difference between Strike Doubling and Die Doubling can be difficult at first. Here are two good references:
This one has a quiz at the end to test your skill:
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