Reversible error in an error by the trier of law (judge) or the trier of fact (jury - or again 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 error 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.

Reversible errors include, but are not limited to:

• Seating a juror who manifested impermissible bias to one party or the other
• Admitting evidence which should have been excluded under the rules of evidence
• Excluding evidence which a party was entitled to have admitted
• Giving an incorrect legal instruction to a jury
• Failure to declare a mistrial when continuing with trial amounts to a denial of due process
• Conversely, granting a mistrial in a criminal case if the defendant objects, unless the grant was necessary to correct manifest injustice

If an appellate court determines that reversible error occurred, it must 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 done.