Rule 15(c) determines when an amended pleading relates back to the date of the original filing. Rule 15(c)(1)(C)(ii) requires that, within 120 days of filing, the later-named defendant "knew or should have known that the action would have been brought against it, but for a mistake concerning the proper party's identity." The plaintiff in Krupski v. Costa Crociere SpA, 2010 U.S. LEXIS 4567 (U.S. June 7, 2010) sued a trade name (Costa Cruises), rather than the appropriate entity (Costa Crociere SPA), whose name appeared on the back of the plaintiff's cruise ticket. The lower courts denied relation back because the plaintiff (1) knew of the existence of the correct entity before the limitations period ran and, therefore, made no "mistake," and (2) unduly delayed seeking leave to amend.

The Supreme Court reversed, holding that "relation back under Rule 15(c)(1)(C) depends on what the party to be added knew or should have known, not on the amending party's knowledge or its timeliness in seeking to amend the pleading." Krupski emphasized that mere knowledge of the existence of the later-named defendant does not preclude a mistake as to its role or status — and that there was no conceivable reason, other than mistake, that one would sue a defendant incapable of providing relief. Note, however, that where an incorrect but viable defendant is named and the correct defendant is then identified to the pleader (as it was here, in an answer), the result could be different. The pleader may be determined to have made a choice, rather than a mistake.