A defect, failure, or mistake in a legal proceeding or lawsuit; a departure from a prescribed rule or regulation.
An irregularity is not an unlawful act, however, in certain instances, it is sufficiently serious to render a lawsuit invalid. For example, a number of states have statutes that require the appointment of a guardian to represent the interests of a child who is being sued. The failure to do so is an irregularity that can be used as a ground for invalidating and setting aside a judgment entered against the child.
In other cases, however, the flaw might be a simple HARMLESS ERROR that can be easily rectified, and, therefore, does not render the proceeding invalid.