Reversible error

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.

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.