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In United States law, a reversible error is an error of sufficient gravity to warrant reversal of a judgment on appeal. It is an error by the trier of law (judge), or the trier of fact (the jury, or the judge if it is a bench trial), or malfeasance by one of the trying attorneys, which results in an unfair trial. It is to be distinguished from harmless errors which do not rise to a level which brings the validity of the judgment into question and thus do not lead to a reversal upon appeal.
A finding of reversible error requires that one or more of the appellant's "substantial rights" be affected, or the evidence in question be of such character as to have affected the outcome of the trial. (See e.g., Montana Petroleum Tank Release Compensation Bd. v. Crumley's, Inc., 174 P.3d 948 (Mont. 2008).) The criteria for determining what constitutes a "substantial right" is somewhat vague however, being that it varies from case to case, each presenting a slightly different interpretation of which rights are essential, or significant enough to warrant this sort of legal protection. Therefore, reversible errors resulting from the violation of an individual's "substantial right(s)" must be considered on an individual basis.
Reversible errors include, but are not limited to:
If an appellate court determines that reversible error occurred, it may reverse the judgement of the lower court and order a new trial on such terms and conditions as are found to be just. Technically, attorney misconduct is not reversible error. Failure of the judge to remedy it during the trial is reversible error. In cases such as unfairly or illegally concealing evidence, there is no error on the part of the court but the court's decision may still be vacated and the matter returned for a new trial, because there is no other way for justice to be granted.