From Cedar Petrochemicals, Inc. v. Dongbu Hannong Chemical Co., 2011 U.S. Dist. LEXIS 4768 (S.D.N.Y. Jan. 14, 2011):

Under Rule 26 of the Federal Rules of Civil Procedure, expert witnesses must submit a written report that includes, among other things, "a complete statement of all opinions the witness will express and the basis and reasons for them [and] the facts or data considered by the witness in forming them." Fed. R. Civ. P. 26(a)(2)(B)***. Rule 26(e) requires that "[a] party who has made a disclosure under Rule 26(a) . . . must supplement or correct its disclosure . . . if the party learns that in some material respect the disclosure . . . is incomplete or incorrect." Fed. R. Civ. P. 26(e)(1). Rule 37(c)(1) provides sanctions for failure to comply with these provisions, holding that a party that fails to disclose or amend is not allowed "to use that information . . . to supply evidence on a motion, at a hearing, or at trial, unless the failure was substantially justified or is harmless." Fed. R. Civ. P. 37(c)(1)***.

Dongbu correctly notes that "experts are not free to continually bolster, strengthen, or improve their reports by endlessly researching the issues they already opined upon, or to continually supplement their opinions." Sandata Technologies, Inc. v. Infocrossing, Inc., Nos. 05 Civ. 9546, 06 Civ. 1896, 2007 WL 4157163, at *6 (S.D.N.Y. Nov. 16, 2007). If an expert's report "does not rely [on] any information that was previously unknown or unavailable to him," it is not an appropriate supplemental report under Rule 26. Lidle v. Cirrus Design Corp., No. 08 Civ. 1253, 2009 WL 4907201, at *5-6 (S.D.N.Y. Dec. 18, 2009) ("Lidle I"). However, "preclusion of an expert report can be a harsh sanction." *** In determining whether preclusion is appropriate, courts must consider: (1) the reasons for the delay in providing the evidence; (2) the importance of the evidence precluded; (3) the prejudice to the opposing party from having to address the new evidence; and (4) the possibility of a continuance. See Softel, Inc. v. Dragon Medical and Scientific Communications, Inc., 118 F.3d 955, 961 (2d Cir. 1997); Outley v. City of New York, 837 F.2d 587, 590-91 (2d Cir. 1988); Lidle I, 2009 WL 4907201, at *6; Point Productions A.G., 2004 WL 345551, at *9. "Before the extreme sanction of preclusion may be used by the district court, a judge should inquire more fully into the actual difficulties which the violation causes, and must consider less drastic responses." Outley, 837 F.2d at 591.