Double Clash
The image to the left is an example of a 1993
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 is an example of a
"Double Clash" because it shows two sets of
reverse details.  

Images courtesy of AgCollector