A more thorough explanation:

Definition: A common error is a mistake that is found in both a copyrighted work and in an allegedly infringing work. This mistake is considered as persuasive evidence of unauthorized copying.

Example: If a book has a spelling mistake on a particular page, and another book that is suspected of being a copy of the first book also has the same spelling mistake on the same page, it is considered a common error. This common error is evidence that the second book was likely copied from the first book without permission.

Explanation: The example illustrates how a common error can be used as evidence of unauthorized copying. If two works have the same mistake, it is unlikely that it is a coincidence. This suggests that one work was copied from the other without permission. Common errors can be used in court to prove that a work has been copied without permission.