Let me identify a middle ground between the slightly competing positions in this case.
Judges Silverman (who writes the majority opinion) and Tashima (who concurs) both agree that a treating physician who becomes a testimonial expert because s/he's expressing opinions at trial beyond the scope of her treatment is required to be designated under Rule 26 and prepare an expert report. I do too. It makes sense. It's consistent with both the text and spirit of Rule 26.
Judges Silverman and Tashima also agree, however, that the law in this area was fairly unclear, and that it's thus not surprising that the plaintiff didn't so designate her treating physician. True as well. Moreover, both agree that the resulting consequence -- that, without this expert testimony, plaintiff didn't have evidence that would survive summary judgment -- seems unfair. It's making a reasonable mistake about unclear law into something dispositive. That's true as well. For this reason, both Judges Silverman and Tashima also agree that while the law is indeed that plaintiff should have designated her experts under Rule 26, they're going to make this principle prospective only, and thus reverse the dismissal below. I'm on board as well.
Where Judges Silverman and Tashima differ is how excited they are about this result. Judge Silverman is fully on board. Judge Tashima goes along only reluctantly.
I'm in the middle. I'll explain why.
Judge Silverman is totally happy with prospective-only application of the resulting rule because he thinks that a contrary application would unfairly penalize the plaintiff. As I said, I get that. But what Judge Silverman's opinion (as well as Judge Tashima's) fails to address are the resulting incentive effects. The reason we give most opinions retrospective effect is because (1) the winning party was "right" on the law, and thus they're "entitled" (as a general legal matter) to the benefit of that law, and (2) because unless we do so, they'll have no incentive to litigate these things. If we tell people: "Yes, you're right, but the law was unclear, so we're only going to apply this rule to people in the future," it's not worth expending tens of thousands of dollars in legal fees (or more) on the underlying battle just to benefit others. Hence no incentive to appeal and to start the process necessary for clarifying the law. That's why we generally make decisions -- even about unclear legal principles -- apply to the parties at hand. Not because we don't recognize that the law is subject to a reasonable dispute. But rather because, institutionally, to do so otherwise would reduce our systemic ability to modify and rationalize and clarify the resulting legal rules. It might be different if we provided everyone with state-paid lawyers who had a duty to file appeals even if it would only benefit others. But we don't. So we need an incentive. That incentive is retrospective application. Something that Judge Silverman doesn't address in his opinion, and that he should care about.
At the same time, I don't want to go overboard on this. I'm not against purely prospective application in every case. It's appropriate sometimes. Including this one. And while I agree with Judge Tashima that purely prospective application should be reserved for the unusual case, I don't agree with his reluctance to find this one precisely such a case. So I'm not entirely sympatico with his opinion either.
For me, what matters here is the actual rule at stake, not just the (allegedly "unfair" or otherwise) results that are engendered by its application. Rule 26 says that when you fail to designate a trial expert, that expert isn't permitted to testify unless the failure to designate was "substantially justified" or harmless. So it's inherently a flexible test; moreover, one that expressly incorporates an inquiry into the justification -- and its reasonable or unreasonable nature -- for the party's conduct. One way, I would hold, that a party could be "substantially justified" in doing what it did was if the law was unclear. As indeed it was here. When that's the statutory (or rule-based) scheme, it makes sense to grant (and apply) more flexibility when deciding whether to apply a new legal principle purely prospectively.
To put it a slightly different way, another -- slightly different -- way that the panel could have resolved this case (but with the same result) would be to say: "We find that, given the uncertainty of the law, plaintiff's failure to designate the expert was indeed substantially justified, so the grant of summary judgment was improper under existing law, and plaintiff is entitled to relief under that law. However, since, as a result of this opinion, the law is now clear that you've got to designate, in the future, a failure to designate will not be substantially justified." So that way you're not just prospectively applying the rule. You're applying it here, but the application of that holding will nonetheless be different going forward.
So it matters to me what the Rule says. Which is why I come out somewhere in between the majority and concurring opinion here. Not reluctantly. But with a view towards both the incentive effects of the principles we apply as well as in light of the actual words used in the rule or statute at issue.
P.S. - Regardless, in the future, designate your experts, okay?