From Ghiorzi v Whitewater Pools & Spas, Inc., 2011 U.S. Dist. LEXIS 125329 (D. Nev. Oct. 27, 2011):

Applying Rule 37(a)(3) and (c)(1) compels the conclusion that an expert's trial testimony should be automatically excluded if a party fails to strictly comply with the requirements of Rule 26(a)(2)(B) unless the court finds that there was a substantial justification for the failure to make complete disclosure, or that the failure to disclose is harmless. Id; Elgas, 179 F.R.D. at 299 (citing Sullivan v. Glock, Inc., 175 F.R.D. 497, 503 (D. Md. 1997)). In the Ninth Circuit, "[t]he district court is given broad discretion in supervising the pretrial phase of litigation . . . ." Continental Lab., 195 F.R.D. at 677 (quoting Miller v. Safeco Title Ins. Co., 758 F.2d 364, 369 (9th Cir. 1985)). If full compliance with Rule 26(a) is not made, Rule 37(c)(1) mandates some sanction, "the degree and severity of which are within the discretion of the trial judge." Keener v. United States, 181 F.R.D. 639, 641 (D. Mont. 1998).

The Ninth Circuit reviews a district court's decision to sanction for a violation of the discovery rules for abuse of discretion which gives "particularly wide latitude to a district court's discretion to issue sanctions under Rule 37(c)(1)." Yeti by Molly Ltd. v. Deckers Outdoor Corp., 259 F.3d 1101, 1106 (citing Ortiz-Lopez v. Sociedad Espanola de Auxilio Mutuo Y Beneficiencia de Puerto Rico, 248 F.3d 29, 34 (1st Cir. 2001)). The burden is on the party facing discovery sanctions under Rule 37(c)(1) to prove harmlessness. Id. at 1107. Exclusion of an expert's testimony for failure to comply with the requirements of Rule 26(a) is a sanction available to the trial court within its wide discretion under Rule 37(c)(1) even in the absence of showing a bad faith or willfulness. Id. at 1106. ***

II. Treating Physicians and Rule 26(a)(2).

Federal Rule of Civil Procedure 26(a)(2)(B) limits required expert witness disclosures to "a witness who is retained or specially employed to provide expert testimony in the case." Fed. R. Civ. P. 26(a)(2)(B). While a treating physician is often called as an expert witness at trial, he or she is not generally retained or specially employed for this purpose, but rather to treat a plaintiff for his or her medical complaints. This district has long recognized that a treating physician may be deposed or called to testify at trial without providing a written report. See Fed. R. Civ. P. 26(a), Advisory Committee Notes to 1993 Amendments. In Piper v Harnischfeger, Magistrate Judge McQuaid rejected a defendant's argument that a treating physician's opinions on matters such as causation, future treatment, extended disability and the like, required compliance with the requirements of Rule 26(a)(2)(B)***>

In a case of first impression in the circuit, the Ninth Circuit recently addressed, when, if ever, a treating physician is required to prepare an expert report compliant with Rule 26(a)(2)(B) in Goodman, 644 F.3d 817. ***[T]he Ninth Circuit held that "when a treating physician morphs into a witness hired to render expert opinions that go beyond the usual scope of a treating doctor's testimony, the proponent of the testimony must comply with Rule 26(a)(2)." Id. at 819-20. ***

In Goodman, the Ninth Circuit recognized that the general rule is that a treating physician is a percipient witness of the treatment rendered rather than an expert retained or specially employed to provide expert testimony. For this reason, a treating physician is ordinarily not subject to the written report requirements of Rule 26(a)(2)(B). Id. at 824 (citing Fed.R.Civ.P. 26(a)(2) Advisory Committee Note (1993)). However, the Ninth Circuit noted that district courts in the circuit have limited treating physician testimony to opinions formed during the course of treatment when the party seeking admission of the testimony disclosed no expert report. Id. at 825. Joining the Sixth, Seventh and Eighth Circuits, the Ninth Circuit held "that a treating physician is only exempt from Rule 26(a)(2)(B)'s written report requirement to the extent that his opinions were formed during the course of treatment. Id. at 826. The court found that the Plaintiff had retained a number of her treating physicians to render expert testimony beyond the scope of the treatment rendered, and that to form these opinions these doctors had reviewed information provided by Goodman's attorney that had not been reviewed during the course of their treatment. For these reasons, the Ninth Circuit agreed with the district court that these doctors fell outside the scope of the "treating physician" exception, and under Rule 26(a)(2)(B), written reports were required.