Capped Die
The image to the left is an example of a capped
die 2001 cent.

A capped die is caused by a coin sticking to the
striking die during the minting process.  When
coins are minted, the planchet rests on one die
while the other strikes it.  When a coin gets stuck
to the upper die it continues to strike other coins.  
The struck coins are called
brockage strikes or
"stuck through die cap" strikes.  The capped die
continues to spread around the outside of the die
until eventually it falls away.

At times additional planchets will get bonded to the
capped die.  

Image courtesy of Heritage Auction Galleries