[LouieFisk]

“Totally not believable in any way shape or form.”
—
I believe, legally speaking, that if a person has given partially false testimony in court, that the rest should be distrusted.
Though someone with a better knowledge of the law re:perjury would be able to correct me if I’m wrong on that.

[Centaur]

I believe you are thinking of the jury instruction "falsus in uno, falsus in omnibus":

"If you decide that a witness deliberately testified falsely about a material point [that is, about a matter that could affect the outcome of this trial,] you may for that reason alone choose to disbelieve the rest of his or her testimony.   But you are not required to do so.   You should consider not only the deliberate falsehood but also all other factors bearing on the witness's credibility in deciding whether to believe other parts of [his][her] testimony."