Gemara

(Rava): "Oh Hoda Elav" obligates bringing a Chatas when he finds out (that he really ate Isur).

The Asham Taluy protects him from punishment (until then).

Kesuvos 22b (Beraisa): If two witnesses say that Ploni (Leah's husband) died, and two say that he did not, she may not remarry. If she remarried, she may remain married.

Question: Perhaps Ploni is still alive. One who has Bi'ah with her must bring an Asham Taluy. How can we let them stay married?!

Answer (Rav Sheshes): She married one of the witnesses who say that Ploni died.

Question: She must bring an Asham. (She is unsure.) How can we let her stay married?

Answer: The case is, she is sure.

Nazir 23a (Beraisa): "He did not know, and sinned" refers to one who ate a piece of Safek Chelev, Safek Shumen (permitted fat). The verse says "he will bear his sin";

Isi ben Yehudah says, there were two pieces of meat in front of him, one of Chelev, and one of Shumen. (Note: Orach Mishor and others fix the text to say that Isi expounds a different verse regarding Asham Taluy "... v'Nislach Lo.")

Had the Torah taught only the case of one piece, one might have thought that only then he gets atonement, for there was no Vadai Isur, but not when there were two pieces, one of which was Vadai Chelev.

Rishonim

Rashi (Nazir 23a DH d'Ikva): When there was Isur Kavu'a, and he knew that one piece is Chelev (one might have thought he does not bring a Korban).

Rashi (Yevamos 88b DH Bari): Asham Taluy is only for one who is unsure. She says that she is sure that he died, e.g. through Simanim.

Rashi (Kesuvos 22b DH b'Omeres): Asham Taluy is only for one who is unsure. She says that she has no doubt, for if he were alive, he would have come (home)!

Question (Ran Kesuvos 9b DH b'Omeres): Rashi was forced to say so, for if she says 'it is clear to me that he died', any woman is believed (even if there are no witnesses that he died)! This is wrong. Surely, if she does not know clearly, she is Chayav Asham Taluy. Rather, she says that she saw him dead. Any woman is believed to say that her husband died if there are no witnesses who contradict her.

Defense (Beis Meir EH 17:42): To answer this, Rashi said that Asham Taluy is only for one who is unsure. Indeed, this permits them to have Bi'ah, but the children will be Safek Mamzerim! Even though the Torah permits a Safek Mamzer, it is astounding that Chachamim would permit to increase Safek Mamzerim in Yisrael. This is not difficult for the Ran. According to him, her testimony (that she knows that he died) is Machshir her children, like we find in Siman 4:26 (when she tells us from whom she is pregnant).

Note: Why is her testimony better than a third witness? Also three (or 100) witnesses against two is a Safek! Perhaps her testimony is better because she acts upon it, and a person is believed about himself more than 100 witnesses (Kerisus 12a). Seemingly, we could apply this to the witness she married (and rely on him more than the opposing witnesses)! Perhaps we do not, for we already rely on him to counter the two witnesses, or because regarding a man we are concerned lest he lies due to his desire for her. However, perhaps she lies, due to fear of being unable to remarry the rest of her life! Also, this is grounds to suspect her more here than in Siman 4:26. There, even if she was pregnant from a Mamzer or Nochri, she could admit and be able to marry Yisre'elim!

Shitah Mekubetzes (Kesuvos 22b, citing Talmidei R. Yonah): The first Tana in Kesuvos holds that when there are two pairs of contradictory witnesses, the witnesses cancel each other, and mid'Oraisa the previous Chazakah (Eshes Ish) applies. Why does the Gemara say that there is Asham Taluy? They are Vadai killed for Bi'ah! Perhaps because she married a witness, and she said that she is sure, we do not leave her on her Chazakah.

Me'iri (Kesuvos 22b DH Kevar): If two witnesses say that Leah was widowed or divorced, and two witnesses contradict them, and she married one of the former witnesses, and she says that it is clear to her that she was widowed or divorced, even though she does not explain how she knows, there is no (Chiyuv) Asham at all. Even if she explains why she is sure, and it is not a solid reason, since she has no doubt, she does not leave. This is like Rashi. Surely if she contradicted the latter witnesses or was Mezim them (say that they were not where they claimed to see the testimony), she does not leave.

Tashbatz (1:1 DH ha'Shelishi): We do not disqualify witnesses until others testify in Beis Din that the former are Resha'im due to an Aveirah (with lashes mid'Oraisa). However, if a woman says that her husband was Mevatel any Moda'ah on the Get (witnesses who say that he was Mevatel the Get), she is sure that the Get is valid. Even if her new husband is not one of the witnesses, if he says that he knows that one of the witnesses of the Moda'ah is Pasul due to an Aveirah, he is like one of the witnesses (who say that the Get is valid).

Poskim

Shulchan Aruch (EH 17:42): If two witnesses say 'Ploni died', and two say that he did not die, his wife may not remarry. If she did, he must leave her new husband, for it is a Safek. If she married one of the witnesses who say that Ploni died), and she says 'I am sure that he died', she need not leave.

R. Akiva Eiger (cited in Pischei Teshuvah 156-157): I learn from Tashbatz (above) that if she married Reuven, and Reuven says 'I know that one of the witnesses who say that Ploni is alive is Pasul', even though Beis Din did not disqualify him and Reuven does not know that Ploni died, this is called that he is sure, and she does not leave.

R. Akiva Eiger (on 43): It is not clear whether a minor is believed to say that she is sure. See Mutzal me'Esh.

Mutzal me'Esh (53): When a woman says that her husband died, we permit her to remarry because due to the severity (13 fines) that apply if she remarries and her husband returns, there is a Chazakah that she checks well (that her really died) before remarrying. Logically, this Chazakah does not apply to a minor. We cannot rely on her actions. However, the Gemara does not distinguish. I did not find anyone who asks this. This requires investigation.

Beis Meir: If her new husband (one of the witnesses) died, and later we found out that Ploni is dead, she may not marry a Kohen, even according to the Ran (that she was truly Vadai). Below (152:3) I proved that we do not rely on her saying that she is Vadai (for herself), but it helps for her children. We must say that it is due to another reason; therefore, perhaps in every case she is permitted to a Kohen.

Beis Meir (152:3): The Ri says that when (there are contradictory sets of witnesses, and) she and the Yavam are Vadai, she does not leave. The Gemara asked that the children are Safek Mamzerim! The Rambam says that there is a Chazakah that they would not ruin themselves, i.e. if they were not sure they would not gave birth to Safek Mamzerim. Perhaps through this, it is like Vadai Heter. Tosfos (Kesuvos 26b DH Anan) relies on the Chazakah that a woman checks before remarrying.

Note: The Rambam's Chazakah does not help if they do not plan to have children, either due to her age or methods of birth control!

Otzar ha'Poskim (339:1): Be'er Heitev (117) brought Rashi and the Ran. The Acharonim hold like Rashi.

Otzar ha'Poskim (312:3, citing ha'Rachash (Chachmas Shlomo?)): R. Yerucham holds that we follow the majority also when a Kosher witness and a Pasul witness oppose a Kosher witness. If so, when two witnesses contradict each other, and she is sure that he died, she should be permitted to all men! We must say that R. Yerucham holds like Rashi, that she can claim that she is sure (but this is not testimony, until she knows that he died).

Otzar ha'Poskim (339:2): Tzemach Tzedek (Chadash 1:86) brings Rishonim who argue about one who married after the witnesses who forbid came. If she married a stranger (not one of the witnesses), we are stringent even if the forbidding witnesses came after she married. However, if she married a witness, we are lenient to say that it suffices if she is Vadai according to Rashi.

Otzar ha'Poskim (338:6): Rashach (2:105) says that if two say that she became Mekudeshes and two refute this, if she married one of the latter witnesses she does not leave. The same applies if she married another man who, before marrying her investigated as much as possible and concluded that there is no concern for Kidushin. He says that we may rely on this like when she married one of her witnesses, for here it is only a Safek mid'Rabanan, for mid'Oraisa the Chazakah is that she is single.

Otzar ha'Poskim (338:7): The Rema (Sa'if 37) says that (if she remarried based on one witness, and later witnesses said that her husband is alive), her new husband is not believed after Nisu'in. Maharatz (150) says that this means that he does not join the first witness. If there are two witnesses on each side (without him), and we do not need him to say that he is Vadai for testimony, rather, just so he will not have a Chiyuv Asham Taluy, he is believed even after Nisu'in. Maharatz brought a proof.

The questioner in Shevet ha'Levi (2:37): In Nazir, Rashi connotes that at the time he ate he knew that it is a Safek. This is astounding. If so, how can he bring Asham Taluy? The Gemara itself connotes like this, for it thought that there is atonement only when there was only one piece. We must say that there are two kinds of Mezid (regarding a Safek). 1) He knows that there is a Safek, and is not concerned for the Isur. 2) He knows about the Safek, but is lenient and assumes that it is Heter.

Question: In Nazir, if he did not know about the Safek when he ate, what was the Hava Amina that there is no atonement when there were two pieces? One brings a Korban if he never knew about the Isur, and found out later that he Vadai sinned, and all the more so if he never knew, and found out later that perhaps he sinned!

Answer (Shevet ha'Levi 2:37): If he will find out that he Vadai sinned, he will bring a Chatas. The Asham Taluy merely protects from punishment, like Rava taught. Perhaps this protection is only when there was only one piece, but not if Ikva Isura! (Then he was more negligent for entering the Safek.) Shitah Mekubetzes in Kesuvos explicitly brings opinions that Asham Taluy is even if he knew about the Safek. There, the Gemara said that one who marries her is Chayav Asham Taluy, before it answered that she married one of her witnesses. We must say that he says that he is sure that Ploni died.