If you've handled a bunch of patent cases without running across the Kessler doctrine, don't feel like the Lone Ranger. Until today, IP wizards like you likely had little cause to know about the patent-claim-killing rule that sprang more than a century ago from Kessler v. Eldred, 206 U.S. 285 (1907). Be warned. It's lethal.

The Kessler doctrine starts where claim preclusion ends and applies only in patent infringement cases. The Supreme Court ruled in Kessler that once a patent holder loses a case against a particular product, that product becomes impervious to future infringement claims.

The doctrine of claim preclusion — what we called res judicata in law school — bars a second suit between the same parties for the same kind of conduct but only for conduct that precedes the judgment in the first suit. Kessler extends the period from the day of judgment until forever (by which we mean the life of the patent).

The Federal Circuit applied the Kessler doctrine in Brain Life LLC v. Elekta Inc., No. 13-1239 (Fed. Cir. Mar. 24, 2014). The panel first ruled that res judicata precluded Brain Life from suing Elekta a second time for infringement of U.S. Patent No. 5,398,684 but only for infringing acts that happened pre-judgment. Kessler covered the infringement that ensued.

But not even Kessler got Elekta home free. It had acquired a new, potentially-infringing product since the judgment in the first case. The court therefore sent the case back to the district court for proceedings as to Elekta's ERGO++ device.