Well, given that the Ben-Hassrath contains women who go out and hunt traitors (Tal-Vashoth), and there were about a dozen episodes of star trek involving the hunter defecting to the side of the hunted, I'd say its more common than we think. After spending years hunting down Tal-Vashoth our hypothetical Ben-Hassrath (BH) will become more and more sympathetic to the plight of those fleeing (as happens in every single one of those star trek episodes) then eventually she flees herself, or she meets a special individual she was ordered to hunt down, and decides instead that he deserves freedom, and decides she does too.
Star Trek references aren't terribly valid for possible scenarios but since I can't think of any real world equivalent to Ben-Hassrath its what I've got.