Die Clash
The image to the left is an example of a 1991
Lincoln Cent that has a been struck from
clashed dies.

A die clash is created when two dies are
damaged by striking each other.  Dies may
strike each other when there is no planchet
between them.  In most cases, the portions of
the reverse image will transfer to the obverse,
or portions of the obverse image will transfer
to the reverse.

The cent to the left shows portions of the
Lincoln Memorial from the reverse that have
transferred to the obverse.   

Images courtesy of Steven Bingham