In what circumstances will a court order that a witness be excluded from testifying in a civil rights action?

In Meyers v. Pennypack Woods Home Ownership Ass'n, 559 F.2d 894 (3d Cir.1977), the plaintiff filed an action under the Civil Rights Act of 1866 and the Fair Housing Act alleging that the defendant discriminated on the basis of race in processing applications to reside at a nonprofit housing complex. Id. at 897. Meyers discovered several new witnesses subsequent to the date that he filed his pretrial memorandum, which contained his witness list. Id. at 903. The district court excluded these witness from testifying. Id. at 904. The appellate court reversed, stating that "[A] reading of the district court's opinion demonstrates how important such testimony might have been and how critical is its absence." Id.