Brockage
The image to the left is an example of Lincoln Cent
Brockage strike.  

A brockage occurs in the following way:   A coin is
struck, then not properly ejected.  A new planchet
is fed and the former coin overlaps the new
planchet.  When the die strikes the coins, a mirror
image of the first coin is sunk into the next coin.  
The initial coin may or may not become a
capped
die.  

Most brockages, like the one to the left, are partial.
Full brockages, where the coins are completely
overlapping, are the most desirable.