An incorrect planchet is when a die of a certain type and/or denomination that strikes a planchet not intended for that type and denomination. It can be a nickel struck on a dime planchet or a half dollar struck on a quarter planchet, or any other combination. You can identify an incorrect planchet by weight and by site if the intended design is larger or smaller than the planchet it is struck on.
Incorrect or wrong planchet errors can be worth $400 for a cent struck on a dime planchet to $18,000 for a 1983 Cent struck on a 95% copper planchet (supposed to be copper plated zinc planchet in 1983). Value will depend on rarity, coin type and denomination as well as which planchet alloy the coin is struck on.
Images courtesy of Heritage Auctions