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. This coin was struck on a 1942 copper blank, since 1943 cents were struck in steel. 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 a few known 1999 SBA Dollars struck on the brass planchet for the 2000 Sacagawea Dollar. A unique specimen is known of a 2000 Sacagawea Dollar struck on a clad planchet for the 1999 SBA Dollar.

Images courtesy of Byers Numismatic Corp.

Significant examples:
1965 Dime struck on a silver planchet, ANACS AU-55 (illustrated above).

Sources and/or recommended reading: