The harmless error doctrine is a legal principle that allows a court to uphold a judgment even if there were errors made during the trial, as long as those errors did not affect the substantial rights of the parties involved. This doctrine emphasizes that not all mistakes during a trial warrant a new trial or reversal of the judgment, particularly if the errors are deemed inconsequential to the outcome of the case. Its application helps to promote judicial efficiency by preventing unnecessary retrials in situations where an error would not have changed the result.