Stop Blawgletter if you've heard this one before.

A lady trips on a cable during an oceanic cruise.  Injuries ensue.  She sues Cruise Company A.  Cruise Company A points the Finger of Blame at Cruise Company B, saying that Cruise Company A merely served as sales agent for Cruise Company B, the actual owner and sailer of the vessel.

The lady drops Cruise Company A and moves to amend her complaint to add Cruise Company B as defendant.  The district court grants her motion.  But Cruise Company B's lawyers — the same ones who got Cruise Company A off the hook — move to dismiss on the ground that the lady waited too long to bring Cruise Company B into the suit.  The district court grants the motion, and the court of appeals affirms.

Yesterday, the U.S. Supreme Court reversed.  The 9-0 Court held that, contrary to the view of the Courts Below, Rule 15(c) allows a party to correct a "mistake" as to the True Defendant's identity even if she knew of the True Defendant's existence.  What matters, the Court concluded, is whether the True Defendant knew or should have known of the gaffe.  "For purposes of that inquiry," the Court noted, "it would be error to conflate knowledge of a party's existence with the absence of mistake."  Krupski v. Costa Cociere, S.P.A., No. 09-337, slip op. at 9 (U.S. June 7, 2010).

You have to like that sentence — calling "absence of mistake" an "error".