Yevamot 88B

והבא עליה באשם תלוי קאי! אמר רב ששת: כגון שניסת לאחד מעדיה.

And as this is an uncertain case, one who has sex with her stands obligated to bring an uncertain guilt-offering. Since before she remarried some witnesses say she is a married woman while others claim she is now a widow, her sex with her second husband involve a possible prohibition that entails karet, and whoever mistakenly performs an action of this kind is liable to bring an uncertain guilt-offering. If so, this second husband must certainly divorce her. Rav Sheshet said: We are dealing with a case where that woman married one of her witnesses, who testified that her husband had died. Since the witness himself has no doubt as to the truth, he is not liable to bring an uncertain guilt-offering.

היא גופה באשם תלוי קיימא! באומרת ״ברי לי״. אי הכי, מאי למימרא? אפילו רבי מנחם ברבי יוסי לא קאמר אלא כשבאו עדים ואחר כך ניסת, אבל ניסת ואחר כך באו עדים ⁠— לא אמר.

The Talmud asks: Even so, she herself stands obligated to bring a uncertain guilt-offering, as she has no personal knowledge of the matter and relied on the witness. The Talmud answers: It is referring to a case where she says: It is clear to me. For whatever reason, she is certain that this is not her husband and that he is dead, and therefore she too is not liable to bring an uncertain guilt-offering. The Talmud asks: If so, what is the purpose of stating this? That is, if Rav is referring only to this particular case, he has not taught anything new, as even R' Menaḥem, son of R' Yosei, stated his opinion only with regard to a case where witnesses came and afterward she married, but in the case where she married and afterward witnesses came, he did not state his halakha with regard to this case.