During trial, the defense counsel make many decisions; sometimes there is an objection to evidence, sometimes not. How the appellate courts deal with allegedly inadmissible evidence depends on whether there was an objection at trial.
If there is an objection the appellate court looks to see if the evidence was objectionable, whether the judge abused his discretion in overruling the objection, and if the error was harmful or harmless (prejudice).
If there is no objection the appellate court may apply the plain error rule.