A mistake as to facts or law. harmless error. A mistake by the judge that does not interfere with a party's rights or remedies, and that therefore does not warrant reversal of the decision. plain error. An error that is so obvious and causes such an adverse effect, that an appeals court reverses a decision despite the affected party's failure to object to it during trial. reversible error. A mistake by the judge that adversely affects a party's rights or remedies, and is, therefore, grounds f...