In what circumstances will a prosecutor be found to have improperly vouched a witness in a criminal case?

(People v. Frye (1998) 18 Cal.4th 894, 971, 77 Cal.Rptr.2d 25, 959 P.2d 183 (Frye ).) However, so long as a prosecutor's assurances regarding the apparent honesty or reliability of prosecution witnesses are based on the facts of [the] record and the inferences reasonably drawn therefrom, rather than any purported personal knowledge or belief, [his] comments cannot be characterized as improper vouching. (Ibid. ) Here, the prosecutor simply advanced the view Munoz was credible based on the evidence, which is permissible.