Definition: An error card is a sports card that contains some sort of mistake, such as a misspelling or a photo of someone other than the athlete named on the card. In previous decades, card manufacturers often corrected errors when they were discovered. This led to situations where one of the cards - either the correct or incorrect version, depending on when the mistake was found and how many of each type got into circulation - sold for substantial premiums on the secondary market. This is not the case today because errors are generally no longer corrected by the card companies.
Examples:
One of Brett Favre's rookie cards is actually an error card because his name is misspelled.