Gemara

(Mishnah): If a man died without children (and Sarah remarried because she was Tzaras Ervah, and the Ervah was found to be an Ailonis, all the fines apply).

Version #1 - Suggestion: The fines apply when Sarah remarried, but if she was Mezanah, she is not fined. This refutes Rav Hamnuna, who says that if a Shomeres Yavam had relations with a stranger, she is forbidden to the Yavam.

Rejection: The fines apply also to Zenus. The Tana discusses marriage, for it is nicer.

Version #2 - Suggestion: The Mishnah discusses when Sarah remarried. The same applies if she was Mezanah. This supports Rav Hamnuna.

Rejection: No, the fines apply to Nisu'in, lest she be confused with a woman who remarried after hearing (false testimony) that her husband died overseas.

(Mishnah): If Leah did Yibum (and Sarah remarried and Leah was found to be an Ailonis, all of the above fines apply to Sarah).

Had we taught only the Reisha, one might have thought we fine only when Yibum was never done; but when Yibum was done, we do not fine her;

Had we taught only the Seifa, one might have thought that she is fined only when she knew that she fell to Yibum, but in the Reisha she thought that she was exempt from the moment her husband died, so she would not be fined.

Yevamos 92a (Mishnah): Leah's husband Yakov and their only son Levi went overseas. They told her that Yakov died and then Levi died. She remarried, and later heard that Levi died first (so she was Zekukah l'Yibum). She must leave her new husband. Her children from him, even after Yakov truly died, are Mamzerim.

(Beraisa): This is like R. Akiva, who says that Ein Kidushin Tofsin b'Chayavei Lavin. Chachamim say that a Mamzer does not result from a Yevamah l'Shuk.

92b (Ameimar): The Halachah follows Shmuel, who says that due to our ignorance, if a stranger was Mekadesh a Yevamah, she needs a Get.

(Rav Ashi): According to this, if her Yavam is a Kohen, she does Chalitzah and she is permitted to her husband.

Objection: We should not allow him to profit from his sin!

Correction: Rather, if her Yavam is a Yisrael, her husband gives her a Get, and she is permitted to her Yavam.

(Rav Gidal citing Rav): Ein Kidushin Tofsin with a Yevamah. Nisu'in takes effect.

Question: If Kidushin does not take effect, neither does Nisu'in!

Answer #1: Rather, she has neither Kidushin nor Nisu'in.

Answer #2: 'She has Nisu'in' like Rav Hamnuna taught, that Bi'as Zenus forbids a Yevamah to her Yavam.

Answer #3: Nisu'in takes effect to require a Get. This is a decree lest she be confused with a woman who remarried based on one (false) witness.

Rishonim

Rif (Yevamos 29a): The Halachah follows Shmuel, requires a Get due to Safek. If her Yavam is a Yisrael and she married l'Shuk (a stranger), he gives her a Get, and she is permitted to her Yavam. Rav taught that Ein Kidushin Tofsin with a Yevamah, but Nisu'in does. I.e. Kidushin does not forbid her to her Yavam, but Nisu'in does, lest she be confused with one who remarried based on one witness.

Question (Ba'al ha'Ma'or): The Halachah follows Shmuel, who says that Kidushin Tofsin with a Yevamah. A Yevamah who married l'Shuk will not be confused with a one who remarried based on one witness. Shmuel agrees that Nisu'in forbids to the Yavam. Why did the Rif bring this in Rav's name?

Answer (Milchamos Hash-m): Also Shmuel forbids her to return to the Yavam lest people say that likewise, one who remarried based on one witness may return to her husband. The Gemara gave only two explanations why a Yevamah may not return after Nisu'in: lest she be confused with an Eshes Ish, or Rav Hamnuna's law (Zenus forbids a Yevamah). Since the Halachah does not follow Rav Hamnuna, we must say the former reason.

Rif: Rav Yehudai Gaon says that if a Shomeres Yavam married l'Shuk without Chalitzah and has no children, she must leave her husband and Yavam with a Get and Chalitzah. She is permanently forbidden to them, lest she be confused with one who remarried based on one witness. If she only accepted Kidushin, if her Yavam is a Yisrael her husband gives a Get and she may do Yibum. She may never marry the Mekadesh, lest a sinner profit. If she married a Yisrael (a stranger) and she has children, she does Chalitzah and stays with her husband. We do not force her to leave, lest people equate her to one who remarried based on one witness, and say that her children are Mamzerim. If she married a Kohen and had children, she does not do Chalitzah. She separates from her husband. She is forbidden to him until the Yavam dies. If her children died without descendants, she does Chalitzah and gets a Get from her husband.

Rif: Some do not rely on this, for the Gemara did not say so. Also, the Yerushalmi brings an argument about a Yevamah who married, and supports the opinion that she must leave our Mishnah (Gitin 80a). If after Sarah married l'Shuk after her Tzarah Leah did Yibum, and then learned that Leah is an Ailonis, Sarah leaves her husband and all the fines apply. The Yerushalmi does not follow the Mishnah to make the children Mamzerim, for this is a minority opinion, but obligates her to leave. This is even if she has children, for it said that they are not Mamzerim! Also I do not rely on the Gaon. Mere reasoning does not override an explicit Yerushalmi. If the Bavli were lenient when she has children, it would say so!

(Rosh Yevamos 10:3): Chachamim hold that there is no Kilkul if a Yevamah married without Chalitzah (Yevamos 94a). This is unlike the Sugya in Gitin. It is, like the first version of Rav Gidal (Nisu'in does not apply to a Yevamah) and the Yerushalmi. Alternatively, since usually the Yevamah does not want the Yavam, we are concerned only for Kilkul mid'Oraisa. (Normally we are concerned even for Kilkul mid'Rabanan.) The Gemara there concluded that we are not concerned even for Kilkul mid'Oraisa, and all the more so Kilkul mid'Rabanan.

Rosh (ibid.): The first version of Rav Gidal says that Nisu'in does not apply to a Yevamah. This is like the Yerushalmi, which says that the Mishnah of a Yevamah who married l'Shuk is unlike Chachamim, who say that Mamzerim do not result from a Yevamah. This is unlike Rav Hamnuna. Since Nisu'in b'Mezid does not forbid her, we do not fine b'Shogeg. When a married woman remarried based on one witness, we fine Shogeg only because b'Mezid she would be forbidden. The Halachah is unlike Rav Hamnuna, but it is like the latter two versions of Rav Gidal, like the Sugya in Gitin. L'Halachah, all the fines apply to Nisu'in b'Shogeg.

Question (Ritva, brought in Nimukei Yosef DH Gemara): It is a stringency to be concerned for confusion with one who remarried based on one witness. How can we be stringent when people will consider her children Mamzerim?!

Answer (Ritva): Chachamim make decrees without distinction.

Rambam (Hilchos Yibum 2:18): If a Yevamah married l'Shuk before Chalitzah and they had Bi'ah, they are lashed and he divorces her, even if she had children. She is forbidden to him and to the Yavam. After Chalitzah she may marry others.

Kesef Mishneh: This connotes that without Bi'ah, she is permitted. However, perhaps he mentions Bi'ah only to teach that they are lashed for it.

Yam Shel Shlomo (Yevamos 10:23): The last two versions of Rav Gidal are like the Sugya in Gitin, therefore the Halachah follows them. We are stringent about a Yevamah who remarried lest she be confused with an Eshes Ish, therefore it applies even if there was Chupah without Bi'ah.

Rambam (20): If a Yevamah l'Shuk had Nisu'in before Chalitzah, and remarried him (after he divorced her and she did Chalitzah), we force him to divorce her, for she is like one who remarried based on one witness. If a Yevamah was Mezanah she does not become forbidden to the Yavam. He may do Chalitzah or Yibum.

Rambam (3:19): A woman's husband and their only son went overseas. They told her that her husband died and then her son died. She remarried. Later, they told her that her son died first. She must leave her new husband.

Poskim

Shulchan Aruch (EH 159:2): If she had Nisu'in with a stranger before Chalitzah, he must divorce her, even if she was Shegagah and they had children. She is forbidden to him and to her Yavam, and the 13 fines (Siman 17:57) apply.

Darchei Moshe (1): The Rivash (482) says that many Ge'onim permit her even if she did Nisu'in, provided that Chalitzah was done later. No one is stringent if there are mere rumors that there is a Yavam.

Note: It seems that the reference should be to Teshuvos Maimoni (Yibum 2:37) - Hagahos Tur ha'Shalem 19.

Chazon Ish (20:6): The Shulchan Aruch says even if she was Shogeg. Tosfos and the Nimukei Yosef say that b'Mezid is not called Nisu'in. It is mere Zenus. The fines do not apply to an Eshes Ish who married b'Mezid. We fine a Yevamah only because she resembles an Eshes Ish! However, this is according to Rav. The Halachah follows Shmuel, who says that Safek Kidushin Tofsin b'Yevamah, so we must be stringent even if she was Mezid.