The common interest doctrine can avoid the normal waiver implications of disclosing privileged communications to third parties. But some courts do not recognize the doctrine at all, and most courts impose various requirements on the doctrine that make it unpredictable and risky.

In Regents of University of California v. Affymetrix, Inc., Case No. 3:17-cv-1394-H-NLS, 2018 U.S. Dist. LEXIS 102554 (S.D. Cal. June 19, 2018), the court dealt with the common interest doctrine's applicability to patent-related communications between the alleged infringer and a separate company that designed and manufactured the pertinent product. Before turning to the adversary's other arguments challenging the doctrine's applicability, the court found the doctrine inapplicable because the manufacturing company's employee was not represented by a lawyer at the time. The court acknowledged that no Ninth Circuit precedent " explicitly requires both parties be represented by separate counsel." Id. at *14. But the court nevertheless explained that "[t]he requirement that each party to a common interest arrangement have an attorney . . . comports with the intent behind the common interest privilege." Id. at *13. Thus, "[w]ithout clearing this first hurdle to invoke the common interest exception, the Court need not reach the remaining arguments." Id. at *16.

Lawyers hoping to rely on the common interest doctrine must keep a careful list of all the pertinent court's requirements. And of course defendants may not know which court will address their common interest doctrine argument until after they have entered into an agreement and shared privileged communications.