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On direct, witness 1 testifies about car crash.

On cross, "Witness 1, are you a drug user?" Witness 1 says he's never done drugs in his life.

Then cross-examiner tries to introduce Witness 2, who says he did drugs with Witness 1 a year ago.

Witness 2's couldn't impeach Witness 1 by contradiction because doing drugs one year ago is a collateral matter.

BUT, could you get Witness 2's testimony in as impeaching capacity? As in, "W1 doesn't even remember what he did 1 year ago, therefore he has a mental defect."

Does impeachment of capacity work in that way? That doesn't seem right, because then you could always get around the collateral issue rule (with regard to impeachment by contradiction) rule by refraining it as a capacity issue.

Thoughts? Am I missing something?

What you are suggesting would most likely not come in for several reasons.

First, even if the issue itself isn't collateral, the fact that it is drug use sounds like a "prior bad act" of the kind that FRE 608 was designed to explicitly keep out.

Second, because it looks like collateral it's going to have a hard time passing the relevance v. probative value test. In general, collateral issues are irrelevant, which is why they're kept out; and because the testimony doesn't go to the truth of the matter asserted, it's hard to say that it's probative.

Third, an attack on capacity usually involves something much more intense than "He did drugs once and doesn't remember now." Instead you would probably need something that looks like character evidence (i.e. "He has a reputation in the community for being extremely forgetful."), or something that looks like expert testimony (i.e. "I have treated this witness for pathological lying for years," as testified by a medical doctor).

All-in-all, it's hard to believe that the testimony, as suggested in the hypothetical, would come in.

Plus, Witness 1 didn't say he didn't remember doing drugs, he said he'd never done drugs. Those aren't the same thing. If a witness denies something, you can't just say the denial means the witness doesn't remember it. There would have to be actual evidence of memory problems.