Gemara

(Beraisa): If one said 'this is your Get from today if I die mi'Choli Zeh (from this sickness)', and he was bitten by a snake, the Get is Pasul. If he said '...if I will not get up mi'Choli Zeh' and he was bitten by a snake, the Get is valid.

Question: Why is the law different in the Seifa than in the Reisha?

Answer (Chachamim of Eretz Yisrael): We did not learn when a lion ate him.

Reuven sold land to Shimon, and accepted Acharayus (responsibility) to compensate him for any Ones. A river was rerouted through the land.

Ravina: He accepted Acharayus for any Ones!

Rav Acha bar Tachlifa and Rava: He did not accept such an exceptional Ones.

Question (Ravina - Beraisa): If he said '...if I do not recover from this sickness' and he was bitten by a snake, the Get is valid.

Counter-question (Rava): The Reisha of that Beraisa says that the Get is Pasul!

Question (Rav Acha mi'Difti): Granted, there is a contradiction within the Beraisa. Does this mean that one cannot ask from it?!

Answer (Ravina): Yes! Since there is a mistake in the Beraisa, we cannot rely on it. We rely on reasoning (he did not accept such an Ones).

Rav Papa and Rav Huna brei d'Rav Yehoshua hired sailors to transport some sesame. The workers accepted Acharayus for any Ones. The river was dammed up. They asked the sailors to pay for donkeys to take the sesame.

Rava: The sailors are exempt. This is an exceptional Ones.

Rishonim

Rif: The Halachah follows Rava (who exempts for exceptional Onsim).

Rosh (6:4): If one thinks about an exceptional Ones, even if he said 'if I die from this sickness', the Get should be valid. He intended to exempt her from Yibum; he meant 'if I die before I recover.' Chachamim of Eretz Yisrael said that we did not learn when a lion ate him, i.e. that it is a Get. The Seifa of the Beraisa is mistaken. Rather, we rely on reasoning. One does not accept exceptional Onsim.

Question (Ran DH Garsinan): According to Rashi (the Rosh's Perush), the Rif should have brought this (that one does not intend for exceptional Onsim). Why did Rav Acha think that we can ask from the Seifa, when the Reisha holds that one intends for exceptional Onsim?!

Answer (Ran citing the Ramban): The Gemara asked about the Seifa. We understand the Reisha even if one intends for exceptional Onsim, for he did not die due to the illness. The Seifa was difficult, for his failure to get up was not due to the illness. If one said 'if I die 'mi'Choli Zeh', we assess whether he died due to that illness (72a). According to Rashi, we should assess whether he recovered before he died! We did not learn when a lion ate him. Even had a common Ones occurred, it would not be a Get, for he did not die due to the illness. Rav Acha thought that even though the Seifa should say 'it is not a Get', we can infer that had he stipulated simply 'if I die', it would be a Get, for one intends for exceptional Onsim. Rava did not answer 'Ones does not disqualify Gitin.' This shows that regular Onsim do not invalidate Gitin, but exceptional Onsim do, just like they do not obligate one in monetary matters.

Rosh (ibid.): However, the Tosefta and Yerushalmi distinguish the Reisha from the Seifa. In the Reisha it is not a Get, for he did not die due to the illness. We did not learn in the Reisha when a lion ate him, i.e. that it is a Get. I do not know why R. Chananel rules like this, against the Bavli. The Rambam rules that in the Reisha it is not a Get, and in the Seifa she is Safek divorced.

Mordechai (360): Ones b'Yedei Shamayim is Ones to invalidate a Get. The Bavli agrees that the Tenai in the Seifa was 'if I will not recover.'

Rambam (Hilchos Gerushin 9:17): If one said 'this is your Get from today if I die from this sickness', and he was bitten by a snake, etc., and died, it is not a Get.

Rambam (18): If he said '...if I do not recover from this sickness' and the house caved in on him, she is Safek divorced.

Ran (ibid.): The Rambam explains 'we did not learn about when a lion ate him' that it is a Vadai Get. Rather, it is a Safek.

Rambam (Hilchos Mechirah 19:5): If Reuven sold land to Shimon and accepted responsibility for any Ones, even if a Nochri stole it due to Reuven, he must pay. However, if the river that used to irrigate it ceased, or the river resumed to pass through and it became a lake, or if wind came and destroyed the land, he is exempt. These are exceptional Onsim. They never crossed the seller's mind when he stipulated. Any exceptional Ones is not included in his Tenai.

Poskim

Shulchan Aruch (EH 145:6): If one said 'this is your Get from today if I die mi'Choli Zeh', and he was bitten by a snake, etc., and died, it is not a Get.

Beis Shmuel (9): The Shulchan Aruch rules like the Rambam, that we are unsure whether or not one has in mind exceptional Onsim. He holds like the Ramban and Ran, that mi'Choli Zeh connotes due to the illness.

Shulchan Aruch (ibid.): If he said '...if I do not recover from this sickness' and the house caved in on him, she is Safek divorced.

Question (Beis Shmuel 10): The Shulchan Aruch (144:1) rules that an exceptional Ones invalidates a Get. If so, why is this a Safek Get? Also according to Rashi, the Get is Batel!

Answer (Eretz Tzvi, in Hagahos Ma'ase Nisim): When he stipulated 'if I will not come', he did not mention death. He expected to fulfill the Tenai, so an exceptional Ones is Mevatel the Get. When he stipulated about death, he wanted to exempt her from Yibum. Maybe he considered exceptional Onsim!

Pnei Yehoshua (Kuntres Acharon, Kesuvos 2b [2] DH Gemara): If a lion ate him, why is the Get Batel? Perhaps he would not have recovered even if not for the lion! Rather, 'we did not learn when a lion ate him' that it is a Vadai Get. Rather, it is a Safek. Rashi agrees, for he said (73a DH Shalchu) 'we cannot say that it is a Get.' The Rambam explicitly says so for this reason, unlike the Magid Mishneh (who says that the Rambam was unsure if the Bavli argues with the Yerushalmi). The Shulchan Aruch rules like this. In 148:1, the Shulchan Aruch discusses exemption from Yibum and says that the Get is Batel, i.e. does not exempt, since it is a Safek.

Shulchan Aruch (CM 225:3): If he stipulated that Reuven (the seller) must pay for any Ones that will occur, even if a Nochri stole it due to Reuven, he must pay.

Gra (3): This is a common Ones, for Nochrim are established robbers - "their mouths speak deceit, and their right hands are false" (Bava Basra 45a).

Be'er Heitev (7): If a powerful Yisrael claimed the sold item, and Beis Din ruled against him, and he took it by force, this is an exceptional Ones. One accepts common Onsim. Poskim argue about whether or not one accepts even semi-common Onsim. If it is a Safek whether the Ones is common, ha'Motzi mi'Chavero Alav ha'Re'ayah. The Rashdam says that devaluation of currency is not common. Others say that nowadays it is common.

Shulchan Aruch (ibid.): If the river that used to irrigate it ceased, or resumed to pass through and it became a lake, or if wind came and destroyed the land, he is exempt. These are exceptional Onsim. They never crossed the seller's mind.

Gra (4,5): Bava Metzia 77a says that it is exceptional for a river (that it is known to cease) to cease. Bava Metzia 105b says that wind is exceptional.

Shulchan Aruch (ibid.): The same applies to all Tana'im of money. We assess the intent of the one who stipulated, and include in the Tenai only familiar things that he stipulated about them, and thought about them at the time he stipulated.

Beis Yosef (DH v'Chosav): The Rashba (4:163) says that even if he accepted 'any Ones in the world that will occur', an exceptional Ones is not included.

R. Akiva Eiger: Teshuvas Beis Yosef (Kidushin 3) says that if one accepted not to claim any Ones in the world, all Onsim are included.

Erech Lechem: If one bought merchandise in order to sell it and pay after a certain time, and he was unusually sick and unable to sell, he can tell the seller 'do not count the time that I was sick, or take back your merchandise.'

Shulchan Aruch (4): A case occurred in which one hired sailors to transport sesame, and they accepted liability for any Ones. The river ceased. This is an exceptional Ones; the sailors need not bring it there through animals. The same applies to all similar cases.

Erech Lechem: If one borrowed money on condition to repay in Venice, and war erupted and he cannot go, the lender must wait. He stipulated knowing that war and bans on entering countries are common.