In legal procedure  misjoinder refers to a wrongful joinder

Joinder

Joinder is a legal term, which refers to the process of joining two or more legal issues together to be heard in one hearing or trial. It is done when the issues or parties involved overlap sufficiently to make the process more efficient or more fair...

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Misjoinder of causes of action, or counts, consists in joining several demands to enforce substantive rights of recovery that are distinct and contradictory.

Misjoinder of parties consists in joining as plaintiffs or defendants persons who have conflicting interests, or who were not involved in the same transaction or event.

Federal Rules of Civil Procedure

According to FRCP Rule 21,