Clashed Die - What is a Die Clash Error?

1955-Franklin-Half-Dollar-Bugs-Bunny-Variety-Close-Up
Bobby131313 / CC-BY-SA-3.0/ PD US money

Definition 

A die clash error happens when the coin dies come together in the coining press without a planchet between them. This causes an imprint of each die to be left on the opposing die face. For example, an impression of the obverse die is left on the reverse die, and vice versa. Although the Mint usually makes an effort to clean up these marks, they are sometimes visible on the coins themselves.

Also Known As

clashed die error

Example:

The Washington Dollar clashed die error was often called the "Extra Spike" type because the clash marks left what looked like an extra spike on Liberty's crown.