Harmless error

A harmless error is a ruling by a trial judge that, although 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. Harmless error is easiest to understand in an evidentiary context. Evidentiary errors are subject to harmless error analysis, under Federal Rule of Evidence 103(a) ("Error may not be predicated upon a ruling which admits or excludes evidence unless a substantial right of the party is affected.") 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.

In the case of Earll v. State of Wyoming 2001 WY 66 29 P.3d 787, the Wyoming Supreme Court distinguished between reversible error (which requires a conviction be overturned) and harmless error (which does not), as follows:

In the evidentiary context, 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.