As noted above, appeals judges generally defer to trial court findings, particularly findings of fact (as opposed to findings of law). Appellate courts resist overruling trial court judgments and provide trial courts with wide discretion in the conduct of trials. "Prefect trials" are not guaranteed. In most cases, an appellate court will overturn a guilty verdict only if the trial court made an error of law that patently or significantly contributed to the trial's outcome. In other words, a trial judge's error will not lead to a reversal of a conviction as long as the error can reasonably be considered harmless. Most errors are deemed "harmless," and there are consequently few reversals of convictions. There are, of course, some types of errors that are so egregious that they are presumed harmful, such as the use of a coerced confession.

Sentencing is a different matter. When a trial court exercises its discretion in sentencing, an appellate court will rarely interfere. In some cases, however, the law specifies a particular sentence; if the judge gets it wrong, the appellate court will usually send the case back for resentencing.