A harmless error is a mistaken evidentiary ruling of a trial judge that, although clearly mistaken, does not meet the burden for a losing party to reverse the original decision of the trier of fact on appeal, or to warrant a new trial. The general burden when arguing that evidence was improperly excluded or included is to show that the proper ruling by the trial judge may have, on the balance of probabilities, resulted in the opposite determination of fact. A harmless error is usually one where the evidence had no relevance to the issues to be decided by the trier of fact, evidence admitted actually helped the party seeking the reversal, or the remaining evidence was overwhelmingly against the party seeking reversal.

For example, a prosecutor may try to bolster its case by bringing in an expert witness to explain the behavior of one of the key witnesses. If the judge allows the expert to testify that there was a reason to explain away inconsistencies in the witness's testimony, this will most likely be grounds for an appeal, as in most cases evidence that only bolsters the credibility of a witness is not admissible . However, if there were a number of other witnesses against the losing party, the appeal court may rule that this mistake was of no consequence and that even if the evidence had been excluded, the losing party would have lost.