What is the test for a motion for a mistrial in a case involving a witness who said something was wrong with another witness?

"The issue here is whether the witness's comment was so incurably prejudicial that a new trial was required. `A mistrial should be granted if the court is apprised of prejudice that it judges incurable by admonition or instruction. [Citation.] Whether a particular incident is incurably prejudicial is by its nature a speculative matter, and the trial court is vested with considerable discretion in ruling on mistrial motions.' [Citation.] A witness's volunteered statement can, under some circumstances, provide the basis for a finding of incurable prejudice. [Citations.]" (People v. Ledesma (2006) 39 Cal.4th 641, 683.)