A more thorough explanation:

A special plea in error is a legal defense used to defeat a writ of error by alleging some extraneous matter. This plea is used in common law and requires the plaintiff in error to reply or demur.

For example, if a defendant is sued for breach of contract and loses the case, they may file a writ of error to appeal the decision. The plaintiff may then file a special plea in error, alleging that the defendant had already released their right to appeal or that the time limit for filing the writ of error had expired.

The special plea in error is a way for the defendant to argue that the writ of error should not be allowed because of some other legal issue that is not directly related to the case itself. This plea requires the plaintiff to respond and either agree or disagree with the defendant's argument.