A collector recently submitted a unique and rare example of an American Silver Eagle mint error coin to NGC for certification.
NGC has confirmed that the error on this coin is due to a die clash.
A die class occurs when the obverse and reverse parts of the die come together without a planchet between them. In some circumstances part of each side of the die is transferred to the other side. This causes part of the reverse to be imprinted on the obverse, and a vice-versa.
Subsequent coins that are struck after the die clash will show signs of the die clash.
According to CoinWeek.com NGC has certified around 20 coins showing less severe die clash errors for the 2017 mintage and even fewer of previous mintages.