Off metals and wrong planchet errors occur when a blank from one denomination is accidentally fed into a press for another denomination. Examples are a nickel struck on a cent planchet and a cent struck on a dime planchet. The coin struck on an incorrect blank will weigh exactly what the denomination of that blank would have been. An even more dramatic wrong planchet error is a coin struck on a previously struck coin of a different metal.

Images courtesy of Byers Numismatic Corp.

Significant examples:
1942 One Cent on a brass Ecuador 10 Centavos planchet.  Struck at the Philadelpha, PA Mint.  An NGC MS-64 example was reported in the October 10, 2000 issue of Numismatic News (page 41), where it was described as being one of only two known.

1943 Bronze Cent 

1943-S Bronze Cent - NGC MS-61 Brown.  Ex - Ira & Larry Goldberg Coins & Collectibles, Inc. February 6-9, 2000. discovered in a Mint-sewn bag of 1943-S steel cents in San Diego, California in the year of issue.

1943-S Bronze Cent.  Ex - Superior Galleries' "Pre-Long Beach Sale", October 1-3, 2000, sold for $60,375.00 - Legend Numismatics

1943-S Bronze Cent.  PCGS VF-35.  Offered for $89,500.00 by The Mint in an advertisement in the July 18, 2000 issue of Numismatic News.  Holder #3457896

2000 Maryland Quarter Dollar struck on a blank for a Sacagawea Dollar.  This was the first instance in U.S. numismatics of a smaller denomination struck on a larger diameter planchet.

Eisenhower Dollar Struck on Philippine 1 Piso Brass Planchet (illustrated above).

Sources and/or recommended reading:
"Maryland Quarter Struck On Dollar Blank", Numismatic News, August 29, 2000, pages 1 and 68