What happens when a planchet fails to enter the die chamber to be struck? It causes a die clash.
You have the anvil die and it's stationary and has the reverse design, then the hammer die with the obverse design, and the obverse design is upside down in orientation to the reverse design.
So when there's no planchet for the hammer die to strike then it strikes the anvil die. When this happens it imparts design features from the opposite die's face.
An example would be a Lincoln Memorial Cent die clash, on the obverse of the coin you would see the pillars of the Memorial (nicknamed Prison Cent) or you might see Lincoln's bust outline over the Memorial on the reverse.
Often this is caught by a mint employee and they will attempt to file the clash marks off the dies, sometimes this creates odd and numerous amounts of die file lines on the die's surface. You will often see these struck on coins as well.
The die filing sessions have also created a few varieties over the years, from missing designer initials or other missing design elements, and a couple are listed here Filed and Over-Used Dies.
To help you identify Die Clash mint errors here are overlay images for each coin type and denomination Lincoln Cent Overlays.
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