ERRORS ON PROOF
This type of error has
only occurred on modern coinage. The Philadelphia Mint prepares the
proof planchets which are then sent to San Francisco or West Point for
striking. A few proof planchets were accidentally mixed in with the
regular blanks, and struck by regular dies. Several 1999 SBA Dollars
have shown up that are either broadstruck or off center on proof
planchets, but struck by regular dies. These coins have
extreme Prooflike fields and a pitted appearance on the unstruck portion
of the blank. These are the characteristics that distinguish this rare and
unusual type of mint error.
Images courtesy of Byers
1999-P Susan B. Anthony Dollar
broadstruck on a Proof planchet (illustrated above).