What is the impact of errors in admitting and denying defense evidence in a case?

Before explaining the reasons for these conclusions, I think it must be emphasized that whatever errors were committed did not--as the majority seems to suggest--merely affect defendant's claim of self-defense which, if accepted, would have resulted in an acquittal. Under all the circumstances such a result was, perhaps, too much to expect even in the absence of error. The case was, however, also submitted to the jury on the theory of "imperfect self-defense"--that defendant harbored an honest but unreasonable belief in imminent peril. (People v. Flannel (1979) 25 Cal.3d 668, 674-680, 160 Cal.Rptr. 84, 603 P.2d 1.) ¹ That "defense," if accepted, would have led to a conviction of voluntary manslaughter. Thus, in evaluating the likelihood of prejudice, we must consider not only the possibility that the admission of the erroneously excluded evidence could have resulted in an acquittal, but also the more realistic prospect that the jury--had it had all [39 Cal.3d 595] the evidence supportive of defendant's version of the incident before it--might have had a reasonable doubt so as to reduce the offense to manslaughter. This "lesser offense" option clearly makes it much more likely that an erroneous exclusion of favorable defense evidence may have affected the outcome.

Now, to the errors themselves.