An appellate court will not reverse for an error that did not affect the outcome. Understanding how harmlessness is assessed shapes both the trial record and the brief.
Civil standard. Generally, the error must affect a substantial right of the party. The burden of persuasion on prejudice is usually on the appellant, though some errors shift it.
Structural errors in criminal cases defy harmless error analysis entirely because they affect the framework of the trial rather than a discrete piece of evidence.
Cumulative error. Multiple errors, individually harmless, may together require reversal. Raising this requires having preserved each of them.
How to argue prejudice. Show what the excluded evidence would have proved and why it mattered to a contested element; show what the jury heard that it should not have and how it connected to the verdict; and address the strength of the remaining evidence, which is the court’s principal concern.
Verdict form consequences. A general verdict supported by multiple theories, one of which was infected by error, may require reversal because it is impossible to know which theory the jury used. Special interrogatories can render an error harmless — which is a reason to think about verdict forms with the appeal in view.
In the trial court, offers of proof and specific objections are what make a prejudice argument possible at all.