Gemara

(Rav Chanan): If witnesses who testified that a Na'arah Me'orasah was Mezanah were Huzmu (others testified that they were elsewhere at the time), they are not killed. They can say 'we did not intend to kill her, only to forbid her to her husband.' (Since they cannot be killed, their testimony cannot kill her.) The case is, they did not warn her. She is a Chaverah (fluent in Halachos), so she need not be warned, according to R. Yosi b'Rebbi Yehudah.

(Beraisa - R. Yosi b'Rebbi Yehudah): A Chaver need not be warned, for warning is merely to distinguish between Shogeg and Mezid.

Question: How can a Chaverah be killed without warning according to R. Yosi?

Answer: She was Mezanah twice (she was already forbidden to her husband after the first time. Surely, the second testimony was to kill her.)

Question: They can say 'we intended only to forbid her to the second Bo'el!'

Answers: She was Mezanah with the same man both times, or she was Mezanah with a relative (she was Asur to him even without their testimony).

Question: Why did Rav Chanan teach about a Na'arah Me'orasah? The same applies after Nisu'in!

Answer: It is a bigger Chidush that the witnesses can say that they only intended to forbid her to her husband, even before she ever lived with him.

Sotah 25a (Mishnah): A widow married to a Kohen Gadol does not drink or receive a Kesuvah.

Inference (Rava): Even though she is forbidden, she can be warned! We must say that she loses her Kesuvah through seclusion only if she was warned!

Rejection (Rav Yehudah of Diskarta): No. The warning causes that seclusion will (permanently) forbid her to the (suspected) Bo'el.

26a (Mishnah): She may be warned about seclusion with any of the Arayos...

One might have thought that "Nitme'ah... v'Nitme'ah" forbids her to her husband and the Bo'el only when divorce or her husband's death would have permitted her to the Bo'el, if not for the Bi'ah. The Mishnah teaches that this is not so.

Rishonim

Rambam (Hilchos Sotah 2:9): If Leah was forbidden to her husband by a Lav or Aseh, and was warned and secluded, she does not drink Mei Sotah. She is forbidden to her husband also due to the seclusion.

Rambam (12): Any woman who was warned and secluded and did not drink for any reason is permanently forbidden to that man. We have a tradition: just like she is forbidden to her husband, she is forbidden to the Bo'el.

Rosh (Kesuvos 1:4): Rav Hamnuna teaches that Bi'ah with a Nochri forbids her to her husband, even though it does not add an Isur to the Nochri.

Mahara Sason (197, cited in Birkei Yosef EH 11:3 DH uv'Reish): This shows that the Isur of the Ba'al and Bo'el do not depend on each other. Therefore, even a woman already forbidden to her Ba'al becomes forbidden to the Bo'el.

R. Zavi (cited in Birkei Yosef DH v'Chazisei): Rava and Rav Yehudah of Diskarta argue about whether we forbid to the Bo'el when she was already forbidden to her husband. Since we rule that seclusion does not cancel the Kesuvah without warning, we can explain the Mishnah like Rava. Warning a forbidden wife is only for the Kesuvah; it does not forbid to the Bo'el. We follow Rava, for the Halachah follows Rava in most places.

Rebuttal (Birkei Yosef): The Rambam and Rosh forbid to the Bo'el. In many places, the Gemara says 'warning is only to cancel her Kesuvah, but not to make her drink', even though it also forbids to the husband or Bo'el.

Shevus Yakov (1:94): The Gemara suggested that we could kill witnesses who testified about the second Zenus of a Na'arah Me'orasah even if they did not warn her. Since she was already forbidden to her husband, surely, they testified to kill her. We asked that they could say that they intended only to forbid her to the second Bo'el. This shows that she becomes forbidden to the second Bo'el, even though she was already forbidden to her own husband.

Rebuttal #1 (the son of the Sha'ar Efrayim): Perhaps this is not the Halachah. The Gemara merely says that we could not kill the witnesses, because they could say that (they erred about the Halachah, and) this was their intent.

Defense #1 (Shevus Yakov): This is a poor rebuttal. If so, according to R. Yosi b'Rebbi Yehudah, how is she ever killed? The witnesses could say that they did not know that she was Mezanah earlier, or (regarding the latter answer) they did not know that the Bo'el was her relative. Rather, we must say that witnesses cannot say that they did not know.

NOTE: We could say that the case is that these same witnesses testified about her first Zenus, and they said now 'she was Mezanah with her brother.'

Defense #2 (Birkei Yosef DH v'Abita): Rashi (Sanhedrin 41a DH u'Farchinan) says that they could say 'we came to forbid her to the second Bo'el', like we hold... Surely, the witnesses did not say these last words! If this law were wrong, Tosfos, R. Yonah, or the Ran would have commented!

Question (Mishnas ha'Levi 15, cited in Pischei Teshuvah EH 178:31): The Gemara discusses Na'arah Me'orasah. If she was Mezanah earlier, she was not a virgin before the current Bi'ah, and her Misah is choking, not stoning!

Rejection (Avnei Nezer EH 235:3): The Gemara did not mention stoning! Rav Chanan discussed an Arusah, for it is a bigger Chidush. The Gemara asked how a woman (not a Na'arah Me'orasah) is killed according to R. Yosi!

NOTE: Perhaps Mishnas ha'Levi holds that we asked why Rav Chanan discussed a Na'arah Me'orasah only after answering how she is killed, to teach that the Misah of Na'arah Me'orasah (stoning) applies. Tosfos (DH Iy) connotes that we answered how a Na'arah Me'orasah is killed.

Answer (and Rebuttal #2 - Mishnas ha'Levi): Rather, first, witnesses testified that she was Mezanah with Dan. The latter witnesses testified about an earlier Zenus with Levi, when she was still a virgin. How could they say that they came to forbid her to Levi? If so, they did not need to say that Levi's Zenus was earlier! Rather, this proves that a second Bo'el is not forbidden, and they needed to say that Levi was really the first Bo'el.

NOTE: The witnesses needed to say when they saw the Zenus. We know that it is earlier! Perhaps he holds that 'Chazrah v'Zinsah' connotes right afterwards; both testimonies were about the same time, so we must say that the latter witnesses explicitly said that Levi's Zenus was first. However, what forced the Gemara to say that one Zenus was right after the other? And if a second Bo'el is permitted, they could say that they came to permit her to Dan! Perhaps this is true; the second answer is that both Bi'os were with relatives. Or, perhaps the first witnesses didn't know Dan, and testified only about her.

Rebuttal (Noda bi'Yehudah 2 EH 20 DH v'Reishis): R. Yosi holds that a Chaver knows the Halachah; it is as if he accepts warning for Misah. This does not apply to her first Zenus. Since she was not warned, the witnesses could not be killed (they could say that they just came to forbid her to her husband), so she knew that she cannot be killed!

Avnei Nezer (ibid., 2): The Gemara said just like she is Asur to her husband, she is forbidden to the Bo'el. It did not say because!

Poskim

Shulchan Aruch (EH 11:1): If Levi was suspected of Bi'ah with a married woman, if she was warned and secluded and she did not drink Mei Sotah, since she became Asur to her husband, she is permanently Asur to Levi.

Pischei Teshuvah (4): This is even if she was Mezanah before and was already forbidden to her husband.

Rema: Similarly, if she was forbidden to her husband due to Levi, she is forbidden to Levi.

Pischei Teshuvah (5): Perhaps the text should be fixed to say like the Be'er Heitev (4), that even a second Bo'el is forbidden. The Mechaber wrote that the Bo'el is forbidden because the husband became forbidden due to him.