What is abuse of a district court's denial of a defendant's motion for a mistrial?

We review for abuse of discretion a district court's denial of a defendant's motion for a mistrial. See United States v. Segal, 852 F.2d 1152, 1155 (9th Cir.1988). A witness's refusal to testify may constitute reversible error when the government makes a "conscious and flagrant" attempt to build its case out of inferences arising from the witness's recalcitrance, or when these inferences add critical weight to the prosecution's case in a form not subject to cross-examination. See Namet v. United States, 373 U.S. 179, 186-87 (1963); Skinner v. Cardwell, 564 F.2d 1381, 1390 (9th Cir.1977).