This summary is not the rule. The rule text controls.
Not every mistake made during a criminal case will change the outcome of that case. If a small error happens during a trial or court proceeding, but it does not actually harm the defendant's rights, the court will simply ignore it.
This means judges and appeals courts look at whether a mistake actually made a difference. If a slip-up in paperwork, a wrong word, or some other minor issue did not affect the fairness of the trial or the result, it will not be a reason to overturn a conviction or restart the case. The mistake has to be serious enough to actually hurt the defendant's rights before a court will do anything about it.
This rule exists so that cases are not thrown out over tiny technical problems that did not really matter. Courts want to focus on real, meaningful mistakes rather than harmless slip-ups that had no true effect on the case.
