Struck Through Die Cap
The image to the left is an example of a 1999
Lincoln Cent that was struck through a late die

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.

A capped die can strike dozens of coins.  The
obverse image of the coins will be very weak.  The
later the die cap, the clearer the obverse image.  
The coins reverse will be struck well, but may
exhibit signs of stress due to the extra pressure of
the strike.  

Coincidently, the cent to the left is also a
AM" variety.