[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."

[LouieFisk]

Yeah, I think that variation of the legal point is pretty much a boilerplate instruction in all jury trials.
As Someone once said:”whoever is dishonest with very little will also be dishonest with much”.