Harmless Error. Whereas plain error is a rule of appellate procedure, harmless erroris a rule of constitutional law which requires that errors affecting the substantial rights of a defendant not be disregarded. When an appellate court concludes that an error occurred during a criminal proceeding, it may still affirm the trial court's judgment by finding that the error was harmless. Harmless error analysis is conducted in order to avoid “setting aside convictions for small errors or defects that have little, if any, likelihood of having changed the result of the trial.”

If the error involved is not derived from the Constitution, it is harmless if it “did not influence the jury or had but very slight effect.”

However, if the error involved is constitutional, the court will determine whether the error was a “structural” or “trial” error, looking “not only at the right violated, but also at the particular nature, context, and significance of the violation.” Structural errors--defects that fundamentally undermine the reliability and fairness of the trial--are not subject to harmless error review and automatically require reversal. Such errors include: (1) denial of the right to a jury trial; (2) racial discrimination in jury or grand jury selection; (3) improper removal of potential jurors, for cause, in capital trials; (4) improper amendment of an indictment; (5) denial of the right to counsel; (6) denial of the right to choice of counsel due to erroneous dis qualification; (7) denial of the right to self-representation at trial; (8) denial of the right to an impartial judge; (9) denial of the right to a public trial; (10) egregious violation of the right to a fair trial; (11) certain discovery violations; and (12) erroneous jury instructions regarding reasonable doubt.

Conversely, trial errors--errors that occur during the presentation of the case--are subject to harmless error review. A trial error is harmless if a court determines beyond a reasonable doubt that the error did not contribute to the verdict. Trial errors include: (1) certain grand jury procedural violations; (2) errors in examination of prospective jurors; (3) variances between the indictment before the grand jury and the proof offered at trial; (4) misjoinder of defendants or offenses; (5) failure to determine whether a defendant's guilty plea is voluntary and that defendant understands the nature of the charges; (6) certain violations of a defendant's rights under the Fourth, Fifth, or Sixth Amendments; (7) absence of the defendant from trial proceedings; (8) juror misconduct; (9) prosecutorial misconduct; (10) improper exclusion of exculpatory evidence; (11) errors in jury instructions; and (12) sentencing errors, including both constitutional and statutory Booker errors. Individual trial errors deemed harmless may require reversal because of their cumulative effect, though curative measures taken at trial often lead an appellate court to find the error harmless.