What is the consequence of an error in admitting evidence in a criminal case?

The erroneously admitted evidence was critical and there can be no doubt its admission was prejudicial. Used as substantive evidence, for all practical purposes it amounted to a confession. Without it, evidence of defendant's participation in the robbery was not strong. It appears reasonably probable that a result more favorable to the defendant would have been reached had the error not occurred. Therefore, reversal of the judgment is required (People v. Watson, 46 Cal.2d 818, 836, 299 P.2d 243).