In law, the term harmless is predominantly used in reference to an appealable error during the course of a trial such as instructions to a jury or an error in a written judgment.
Harmless errors are not enough to justify a successful appeal of such a tainted decision.
For example, in US v Gomez, Justice Pryor used the words "reasonable doubt":
"A district court's failure to instruct a jury on all of the statutory elements of an offense is subject to harmless-error analysis. The failure to instruct a jury on an essential element of an offense is harmless when it is clear beyond a reasonable doubt that a rational jury would have found the defendant guilty absent the error."
In Jones v Caines, Madam Justice Clement of the same court deviated slightly from the standard by referring to otherwise overwhelming evidence of guilt:
"An error is harmless, however, when the evidence of the defendant's guilt is overwhelming.
"The state bears the burden of showing that the error was harmless."
Again, yet another standard by the United States Court of Apeals, in Padilla (2009); this time highly probable:
"A non-constitutional evidentiary error is harmless if it is highly probable that the error did not influence the verdict.
"The government bears the burden of establishing harmless error.
"The harmless error inquiry is case-specific. Among other factors, it requires consideration of the centrality of the tainted evidence, its uniqueness, its prejudicial impact, the use to which the evidence was put, and the relative strengths of the parties' cases."
Finally, though it is clear that the threshold of an error, for it to be found harmless, is high, judicial confusion continues with this, from US v Johnson (Justice Wilkinson):
"Any error, defect, irregularity, or variance that does not affect substantial rights must be disregarded.
"Erroneously admitted evidence is harmless if a reviewing court is able to say, with fair assurance, after pondering all that happened without stripping the erroneous action from the whole, that the judgment was not substantially swayed by the error."
- Jones v. Cain, 600 F. 3d 527 (United States Court of Appeal, 2010)
- US v. Gomez, 580 F. 3d 1229 (United States Court of Appeals, 2009)
- US v Johnson, 587 F. 3d 625 (2009, United States Court of Appeals)
- US v. Pakala, 568 F. 3d 47