Gemara

30a: A case occurred in which Reuven told his wife 'this Get will take effect if I do not appease you within 30 days.' He tried appeasing her; she was not appeased.

Version #1 (Rav Yosef): He did not give to her a chest full of coins (i.e. he did not try hard enough to appease her. The Get takes effect.)

Version #2 (Rav Yosef): Must he give to her a chest full of coins?! He stipulated only to try properly to appease her. He did so. She refuses to be appeased!

Version #2 is like the opinion that Yesh Ones b'Gitin (to be Mevatel them. I.e. if one gave a Get on Tnai (conditionally), and the Tnai was fulfilled due to Ones, it is not a Get.) Version #1 is like the opinion that Ein Ones b'Gitin.

34a: A man told his Arusah 'this Get is valid if I do not make Nisu'in by Rosh Chodesh Adar.' When the time came, he said 'I meant Rosh Chodesh Nisan!'

The Get is not Batel due to the Ones. Ein Ones b'Gitin!

73a (Rava): If a seller accepted responsibility to compensate for any Ones, this does not include exceptional Onsim. (He did not intend for such Onsim.)

Question (Ravina - Beraisa): If one said 'this is your Get from today if I will not get up from this illness' and he was bitten by a snake, the Get is valid.

Counter-question (Rava - Reisha): If he said '...if I die from this illness', and he was bitten by a snake, the Get is Pasul! (The Beraisa is erroneous.)

Kesuvos 2b - Version #1: Rava holds that Ein Ones b'Gitin.

Question: What is his source?

Answer #1 (Mishnah): If one gave a Get and said 'this is your Get if I don't come within a year' and he died during the year, the Get is invalid.

Inference: Had he only fell sick, the Get would be valid!

Rejection: (We may not infer this.) The Mishnah teaches unlike Raboseinu (who say that the Get took effect from that day, before the husband died.

Answer #2 (Seifa): If he said 'this Get is valid take effect from now if I don't come within a year', and he died during the year, the Get is valid.

Inference: Also if he got sick, the Get would be valid!

Rejection: It is valid only if he dies. He intended to exempt his wife from Yibum.

Answer #3: A man gave a Get on condition that he not return within 30 days. He was unable to return on day 30, because the ferry was on the other side. Shmuel ruled that the Get is valid.

Rejection: A common Ones is different. Since he should have stipulated about it, but did not, he accepted whatever will happen.

Answer #4: Rava's law is a decree due to pious and immoral women. Mid'Oraisa, if a Tnai was fulfilled due to Ones, the Get is invalid. Pious women might refrain from remarrying, from concern lest there was an Ones. Immoral women who know of an Ones might deny it and remarry; their children would be Mamzerim. Therefore, Chachamim (uprooted the Kidushin and) decreed that the Get is valid.

Version #2 (Rava): Also regarding Gitin, Ones exempts (i.e. the Get is invalid).

Answers 1-3 are asked against Rava, and we answer like the rejections above.

Rishonim

Rif and Rosh (15a and 3:10): Rav Yosef's teaching and the man who said that he will marry by Rosh Chodesh Adar show that the Halachah is Ein Ones b'Gitin.

Mordechai 360 and Hagahos Ashri: Ra'avan rules that Ein Ones b'Gitin even for Ones b'Yedei Shamayim. Sefer ha'Miktzo'os rules that Yesh Ones b'Gitin b'Yedei Shamayim (like the Reisha says when a snake bit him), but Ein Ones b'Gitin b'Yedei Adam. A Gaon is stringent for Version #2 in Kesuvos, which says that Yesh Ones b'Gitin.

Question (Beis Yosef EH 144 DH v'Chasuv): The Mordechai rules like Sefer ha'Miktzo'os that Yesh Ones b'Yedei Shamayim, and cites a Gaon who is stringent for Version #2. The Mordechai must hold like Version #2! In Version #1, Rava holds that illness, which is b'Yedei Shamayim, is not Mevatel a Get!

Answer (Gra 144:1): The Gaon is stringent like both versions. (Sefer ha'Miktzo'os rules totally like Version #2 - Hagahos in Tur ha'Shalem (4).)

Rambam (Hilchos Gerushin 9:8): If one gave a Get on condition that he not return within 30 days, and was returning but got sick or was delayed by the river, it is a Get even if he screams that he is Anus, for Ein Ones b'Gitin.

Rosh (Kesuvos 1:2): R. Chananel rules that Ein Ones b'Gitin. Many supports were brought for this. We do not rely on the poor answers given. Gitin 34a supports this. However, in Gitin 73a Rava says that an exceptional Ones disqualifies a Get. Here, in Version #1 Rava holds that even a rare Ones does not disqualify a Get! If not for R. Chananel's ruling, we could say that Gitin 73a holds like Version #2 in Kesuvos. It seems that there are three levels of Onsim. The concerns for pious and immoral women do not apply to exceptional Onsim. All agree that such Ones is Mevatel a Get. All agree that a common Ones, like missing the ferry, does not disqualify a Get. They argue about semi-common Onsim. The Halachah follows Version #1; it is not Ones.

Ran (Gitin 35a DH Garsinan): Rava did not answer Ein Ones b'Gitin. This shows that regular Onsim do not invalidate Gitin, but exceptional Onsim do.

Poskim

Shulchan Aruch (EH 144:1): If one said 'this is your if I will not return within 30 days', and he was returning but got sick or was delayed by the river and came after 30 days, it is a Get. This is even if he screams that he is Anus, for Ein Ones b'Gitin and he did stipulate about Ones. He did not think about an exceptional Ones, e.g. a lion eating him, so it is Mevatel a Get and she falls to Yibum.

Beis Yosef (DH Af): Ge'onim hold that b'Yedei Shamayim is Ones to be Mevatel a Get. We must say that the man returning could not get a ferry to cross the river. If he was detained because the river was overflowing, this is b'Yedei Shamayim and Yesh Ones b'Gitin.

Beis Shmuel (1): If the Get was contingent on her doing something for him, Ones does not help, for he consented only if she will fulfil (Bach DH k'Shem). Therefore, after R. Gershom's enactment that one may not divorce a woman against her will, even an exceptional Ones does not exempt (Rosh 46:2)!

Rebuttal (Beis Meir DH u'Mah) The Rosh says that if pardoning a Tenai makes the Get take effect earlier than she wants, it is valid, but he transgressed R. Gershom's decree!

Beis Shmuel (2): The Shulchan Aruch does not distinguish between b'Yedei Shamayim and b'Yedei Adam.

Rema: Captivity is a common Ones. The Get is valid, since he did not stipulate.

Chelkas Mechokek (1): We find that captivity is uncommon (Bava Metzia 94b), and the Rema rules like this (CM 176:48). However, it is not totally uncommon, therefore the enactment Ein Ones b'Gitin applies to it.