Definition: Subtle differences in design details can differentiate dies used in different years.  Whether accidental or purposeful, obverse dies are sometimes mated with a reverse die meant for a previous or subsequent year.  These are often called “transitional reverses”.  Well-known examples include 1992(P) and 1992-D Lincoln cent obverses mated to a 1993 reverse.

The U.S. Mint normally takes a bit more care when they are producing proof and bullion coins.  The One Dollar Silver Eagle bullion coin had a relatively mistake-free history until 2008.  In that year, one or more 2008 silver eagle obverse dies were mated with a 2007 reverse die.

The most obvious difference between the two dies is found in the style of the U of UNITED.