From Jalbert v. Grautski, 2008 U.S. Dist. LEXIS 119268 (D. Mass. Mar. 31, 2008):

Under Fed. R. Civ. P. 37.1(c), a party's failure to disclose information or identify a witness as required by Rule 26(a) will normally result in the party being barred from using that information or witness at the trial, unless the failure was substantially justified or is harmless. See Klonoski v. Mahlab, 156 F.3d 255, 269 (1st Cir. 1998) ("[T]he required sanction in the ordinary case is mandatory preclusion."). However, the Court may excuse a failure to disclose where the failure is justified or harmless. See Fed. R. Civ. P. 37.1(c). The Court must bear in mind the intent of the disclosure rules "to facilitate a fair contest with the basic issues and facts disclosed to the fullest practical extent." Poulis-Minott v. Smith, 388 F.3d 354, 358 (1st Cir. 2004) (internal quotation and citations omitted). As the First Circuit has explained:

'Trial judges must work a complicated equation, balancing fairness to the parties with the need to manage crowded dockets.' This means that the court of appeals must consider a multiplicity of pertinent factors, including the history of the litigation, the proponent's need for the challenged evidence, the justification (if any) for the late disclosure, and the opponents's ability to overcome its adverse effects. Surprise and prejudice are important integers in this calculation. So too is an assessment of what the late disclosure portends for the court's docket.

Gagnon v. Teledyne Princeton, Inc., 437 F.3d 188, 197-98 (1st Cir. 2006) (quoting Macaulay v. Anas, 321 F.3d 45, 51 (1st Cir. 2003)). ***

Notwithstanding plaintiff's unexcused failure to comply with the rule, the Court concludes that exclusion of the experts' opinions is not an appropriate sanction under the circumstances. No trial date has been set in this matter, and therefore the Court may require plaintiff to provide the required disclosures without causing undue delay. Furthermore, plaintiff does not have a history of discovery abuse in this litigation. Accordingly, at this stage in the litigation, the Court is persuaded that it is appropriate to allow plaintiff 30 days in which to supplement his disclosures for Dorny and Morrison. See Diomed, Inc., v. AngioDynamics, Inc., 450 F. Supp. 2d 130, 136-37 (D. Mass. 2006).

The Court cannot, however, simply ignore the fact that plaintiff ignored a basic discovery obligation without justification or excuse. Plaintiff accordingly will be required to reimburse defendants for all costs, expenses, and attorneys' fees reasonably incurred in filing the motion to exclude and the motion to join, as well as any other costs, expenses, or attorneys' fees reasonably attributable in any way to plaintiff's failure to provide expert discovery. Furthermore, and obviously, any future violation will meet with harsher sanctions. 3