658, 660 (2d Cir.1987). But we rejected the approach of these circuits in Bowie, 232 F.3d at 933 n. 7, reasoning that any special concerns were properly addressed in balancing probative value against prejudice, etc. under Rule 403. Bailey attempts to distinguish Bowie on the ground that there the corroboration was of the act actually charged in the indictment, while in this case, the corroboration is only of other acts admitted under 404(b) (Clayton’s prior seven to twelve cocaine deals with Bailey). But Bowie itself makes no such distinction, and no language in Rule 404(b) supports it.
Nonetheless, use of 404(b) evidence for corroboration does have inherent complications. Corroboration, in and of itself, is not a separate purpose belonging in the open class of permissible purposes referred to in Rule 404(b)’s second sentence. If it were, evidence could slide past the rule against improper character evidence. To decide if Rule 404(b) evidence is admissible for corroboration, the court must-determine what is being corroborated and how. If similar past acts were corroborative only because they showed the defendant’s character and the likelihood of “action in conformity therewith,” plainly the rule would call for" exclusion. On the other hand, evidence might corroborate a witness’s testimony by showing plan, purpose, intent, etc. and therefore be admissible under 404(b). The label “corroboration” thus merely invites a closer look at exactly how the evidence may be probative.
Clayton, it will be recalled, was not himself a part of Bailey’s two New York drug deals. The only legitimate way for that evidence to corroborate Clayton’s testimony was to help show Bailey’s knowledge, intent, etc. as to the seven to twelve Rule Wi(b) drug deals that Clayton mentioned. In other words, they were 404(b) evidence for Clayton’s 404(b) evidence.
As to Bailey’s intent or purpose in the seven to twelve incidents, it is hard to see how the New York deals could add. Clayton had summarily described the deals, making clear that they involved cocaine and that Bailey was selling to him, see 4 Trial Tr. 7-11, but Clayton had never in any way put Bailey’s state of mind in issue. Bailey could have been a zombie throughout the seven to twelve deals, and the fact would not have contradicted Clayton’s testimony in the slightest. But the incidents do help to show that Bailey had opportunities to acquire cocaine, and at least the 1998 incident (involving some semblance of a drug deal rather than mere possession) seems relevant to show knowledge of how to deal in cocaine.
In the weighing required by Rule 403, however, the evidence had little probative value for permissible corroborative purposes. Given the vague nature of Clayton’s testimony about the seven to twelve past deals with Bailey, jury interest in the sort of opportunity and knowledge described was likely minimal. On the other side, by contrast, there were risks of unfair prejudice, confusion and misleading the jury. The permissible link is subtle, the impermissible one obvious and likely to be salient. The likelihood of confusion and prejudice seems overwhelming, at least in the absence of a clear and emphatic instruction, the framing of which strikes us as a daunting task. In any event, no such instruction was even attempted here, rendering the corroboration instruction error.
But this error was harmless. The incremental effect of the corroboration instruction was likely very small. It seems improbable that it did much to strengthen the jury’s belief in Clayton, especially as the evidence of the New York incidents had already been properly admitted. And