In season 1 of Homeland,
Carrie knows that Brody can trick the lie detector, because of the last question (she has slept with him, he lies about it, but the lie detector doesn’t recognize it as lie).
She can’t tell it someone at this point, because she doesn’t want to disclose her relationship with Brody.
Later she discloses the relationship (at least to Saul). At this point, all the others (including Saul) still don’t believe that Brody is a terrorist.
Why doesn’t Carrie tell them (or only Saul) at this point about Brody’s tricking of the lie detector? Of course it wouldn’t prove that he is a terrorist, but it could be an important evidence.