concurring in part and dissenting in part:
I agree with the Court that 14 D.C. Code § 305, as applied to defendants whose offenses were committed prior to its effective date, is an ex post facto law. I concur, therefore, in remanding the cases to allow the District Court to consider, under the Luck rule,1 whether evidence of prior convictions should have been admitted.
My disagreement with the Court concerns only its holding in the Henson *50case, contained in footnote six of the Court’s opinion, that it was harmless error in the circumstances of the case for the trial court to fail to give, sua sponte, an immediate cautionary instruction regarding the limited purpose for which evidence of former convictions, may be used. The Court says that, in any event, it does not find this omission cognizable as plain error.
There is no question, of course, that if evidence of Henson’s prior Harrison Act conviction was admissible at all, it was admissible only for the purpose of evaluating the credibility of his testimony in his own behalf. Nor is there any question that such evidence, when admitted, can have a significant prejudicial effect, particularly when the former conviction was for an offense similar to that for which the defendant is being tried. When it decides to admit such evidence, the trial court should be aware, without prompting of counsel, of the special dangers posed to the fairness of the trial. It is, in my view, incumbent upon the court to take whatever steps necessary to mitigate these dangers as much as possible. And while the immediate cautionary charge is, I grant, a fragile reed upon which to rely, it is the best device available for reducing the possible prejudicial effects of prior conviction evidence.
Despite its erosion in subsequent cases,2 I would adhere to the rule stated in McClain v. United States: 3
We would hold that whenever evidence is admitted only for a limited purpose, it is plain error, in the absence of manifest waiver, to omit an immediate cautioning instruction. The danger of prejudicial effect from such evidence is so great that only an immediate and contemporaneous instruction can be considered sufficient to protect defendants. As long as we continue to have rules of evidence which admit testimony for some purposes but not for others, we must guard against its misuse by the jury.
Moreover, I cannot agree that the trial court’s failure to abide by this rule was, in the Henson case, harmless error. The Court recognizes the “inescapable remoteness of appellate review;”4 yet it would attempt to estimate the quantum of benefit that might have been derived had the immediate instruction been given in this case. The Court may well be correct in its conclusion that, in fact, the quantum of benefit would have been negligible. But, of course, its conclusion rests entirely and necessarily on speculation, the premises of which are uncertain at best.
Such speculation may often be a necessary, if unfortunate, aspect of the appellate function, and I have no doubt that the harmless error rule may be appropriately applied, particularly when an error concerns matters peripheral to the central issues in a case. Here, however, I think the Court’s enthusiasm for the harmless error rule is altogether misguided. The requirement of an immediate cautionary instruction is intended to mitigate a substantial danger to the fairness and accuracy of the guilt-determining process. In my view, it profits neither the cause of judicial efficiency nor what is more important, the administration of justice, to announce a rule and then to undercut it by repeatedly pronouncing its violation harmless. Reluctant as we are to “overload the system” by requiring that a case be retried, by refusing to do so we may well defeat the goal for which we strive. It is, I think, precisely our readiness to find an error harmless that makes it likely to recur and thus to be again and again the *51subject of our review. Adherence to a straightforward rule may do much to ensure consistency in our administi'ation of justice and, in the long run, to promote its efficiency.
With the exception of this point, I concur in the judgments and the opinion of the Court.
. Luck v. United States, 121 U.S.App.D.C. 151, 348 F.2d 763 (1965).
. See, e. g., United States v. Fench, 152 U.S.App.D.C. 325, 470 F.2d 1234 (1972) ; United States v. Thomas, 148 U.S.App.D.C. 148, 459 F.2d 1172 (1972) ; United States v. Mizzell, 146 U.S.App.D.C. 399, 452 F.2d 1328 (1971) ; United States v. Bobbitt, 146 U.S.App.D.C. 224, 450 F.2d 685 (1971).
. 142 U.S.App.D.C. 213, 440 F.2d 241, 246 (1971) (emphasis supplied). See United States v. Gilliam, 157 U.S.App.D.C. 375, 484 F.2d 1093 (1973).
. Luck v. United States, supra note 1, 348 F.2d at 769, quoted in the majority opinion, supra, at 1308.