In law, a reversible error 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 that do not rise to a level that bring the validity of the judgment into question and thus do not lead to a reversal upon appeal.
A finding of reversible error requires that substantial right of the appellant 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)
Reversible errors include, but are not limited to:
If an appellate court determines that reversible error occurred, it may reverse the judgment 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.