This is a unique discovery and first reported in Mint Error News. Recently a handful of blank planchets used to strike the Presidential Dollars were discovered in a bag from the U.S. Mint. In it were the following two mint errors, including a newly discovered mint error type. These two blanks were sent to NGC for authentication and certification. The first is a blank planchet with a defective clad layer. The other blank which is the newly discovered error type is a planchet with a burnishing ball wedged into the clad layer.
Burnishing balls are used in the minting process to make the surface of the coin blank brighter and to remove any surface scaling or discolouration. A finishing machine is used that tumbles the coin blanks with a mixture of stainless steel balls and other media like ballcones and special chemicals, which etch and polish and brighten the surface.
This process more than likely caused the damage on the planchet with the defective clad layer authenticated by NGC. These coins are currently available for sale on my website, alscoins.com