What is the harmless error rule for all penalty trial errors?

The California harmless error rule for all penalty trial errors is stated by People v. Hines, Supra, 61 Cal.2d 164, 37 Cal.Rptr. 622, 390 P.2d 398. In that case, we examined the unique aspects of penalty trials which render inapposite the 'reasonably probable' harmless error rule which is applicable to guilt errors. Because of the absolute discretion afforded a jury presented with 'a mass of material,' because '(t)he precise point which prompts the penalty in the mind of any one juror is not known to us and may not even be known to him,' and because "(t)o attempt to assess the effect of error in this legal vacuum is to superimpose one untestable surmise upon another," we reaffirmed the rule that "any substantial error occurring during the penalty phase of the trial, that results in the death penalty, since it reasonably may have swayed a juror, must be deemed to have been prejudicial." (Id., at pp. 168--170, 37 Cal.Rptr. at p. 626, 390 P.2d at p. 402, and cases cited.)