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 proof-like 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.