A transitional error occurs when a coin is struck on a planchet from a previous year with different metal composition. The most famous transitional is a 1943 copper cent struck on a 1942 copper blank. 1943 cents were struck in steel because of the copper shortage during World War II. Other famous transitionals include 1965 coinage struck in silver instead of clad.
There are also transitionals struck on blanks for the next year. An example is 1964 coinage in clad instead of silver. Most recently, transitionals were discovered involving the SBA and Sacagawea Dollars of 1999 and 2000. There are eight known 1999 SBA Dollars struck on the brass planchet for the 2000 Sacagawea Dollar, and four known 2000 Sacagawea Dollars struck on a clad planchet for the 1999 SBA Dollar.