A federal law tells American appeals courts to decide cases “without regard to errors or defects which do not affect the substantial rights of the parties.” The rule for federal criminal trials is shorter. Such an error “must be disregarded.” In 1967 the Supreme Court noted that all 50 states had similar rules.
In 1946 the Supreme Court explained where the rule came from. It quoted a trial judge who said appeals courts towered “as impregnable citadels of technicality.” The Court added that in many jurisdictions a criminal trial had become “a game for sowing reversible error in the record.”
In that case the Court asked whether the error had “substantial and injurious effect or influence in determining the jury’s verdict.” Under the rule, showing a mistake is not enough. Some rights are an exception. The Supreme Court has called them “so basic to a fair trial” that a violation “can never be treated as harmless error.”