Is evidence of past criminal conduct not improperly affecting the jury's deliberations?

Evidence of past offenses may not improperly affect the jury's deliberations if the facts are equivocal, the charged offense is dissimilar, or the evidence is obviously used to affect one or more of the many legitimate purposes for which it can be introduced. (See Evid.Code, 1101, subd. (b); People v. Haylock (1980) 113 Cal.App.3d 146, 150, 169 Cal.Rptr. 658.) Neither precedent nor policy favors a rule that would saddle the trial court with the duty either to interrupt the testimony sua sponte to admonish the jury whenever a witness implicates the defendant in another offense, or to review the entire record at trial's end in search of such testimony. There may be an occasional extraordinary case in which unprotested evidence of past offenses is a dominant part of the evidence against the accused, and is both highly prejudicial and minimally relevant to any legitimate purpose. In such a setting, the evidence might be so obviously important to the case that sua sponte instruction would be needed to protect the defendant from his counsel's inadvertence. But we hold that in this case, and in general, the trial court is under no duty to instruct sua sponte on the limited admissibility of evidence of past criminal conduct. ¹⁹