Rule 86.Harmless Error

Rule 86 is one sentence long in substance, and it governs how every other rule in Part X is applied.

Unless justice requires otherwise, an error in admitting or excluding evidence -- or any other error by the court or a party -- is not grounds for granting relief under Rule 83, or for vacating, modifying or otherwise disturbing a judgment or order. At every stage, the court must disregard all errors and defects that do not affect any party's substantial rights.

What it does to Rule 83

Rule 83(a) lists eight grounds for altering a judgment, several of which are errors on their face -- error in admitting or rejecting evidence, irregularity in the proceedings, other errors of law. Rule 83 already qualifies each with "materially affecting a party's rights," and Rule 86 states the same requirement from the other direction and extends it to every route by which a judgment might be disturbed. Establishing that something went wrong is not enough; a party must show it mattered.

Two features worth noting

The rule reaches errors by a party, not only by the court. And it is written as a duty: the court must disregard errors and defects that do not affect substantial rights, at every stage of the proceeding. It is not a discretionary tolerance for small mistakes but an instruction to set them aside.

The opening qualifier -- unless justice requires otherwise -- keeps the rule from being absolute, and is the only avenue for arguing that an error should matter even where the effect on a substantial right is hard to demonstrate.