Gemara

R. Yochanan taught that one who swears 'I will not sleep for three days', we lash him (for a vain oath) and he may sleep immediately!

Sanhedrin 72b (Beraisa): "Damim Lo" (one may not kill a thief who surely would not kill the Ba'al ha'Bayis, e.g. one tunneling into his son's house), on a weekday or Shabbos.

Question: If one may not kill on a weekday, all the more so on Shabbos!

Answer (Rav Sheshes): This teaches that if the ground caved in on him we unearth him to save him, even on Shabbos.

Shabbos 19a (Beraisa): One may not embark on a ship less than three days before Shabbos. If he goes for a Mitzvah, it is permitted.

(Beraisa): We may not besiege a Nochri city less than three days before Shabbos. If we did, we may continue the siege on Shabbos.

45a: Rav was asked if one may remove a Ner Chanukah lest it be seen by Chaverim (a nation that decreed about Neros). He permitted it.

Question (Rav Kahana and Rav Asi): Is this really the law?

Answer (Rav): In Sha'as ha'Dechak (pressed circumstances), we may rely on R. Shimon's opinion (who permits Muktzah).

Eruvin 67b: A case occurred in which a baby was due to be circumcised on Shabbos, and the hot water prepared to bathe him spilled. Rabah told people to tell a Nochri to bring the water from Rabah's house.

Question (Abaye): Haza'ah (sprinkling Mei Chatas to Metaher) is Shevus (forbidden mid'Rabanan on Shabbos), just like commanding a Nochri (e.g. to carry without an Eruv). Just like we do not permit Haza'ah for the sake of Korban Pesach, one may not command a Nochri for the sake of Milah!

Answer (Rav Yosef): We are more lenient about Shevus without an action (carrying without an Eruv) than Shevus with an action (Haza'ah).

Rishonim

Rif and Rosh (Shabbos 7a and 1:38): One may not leaving on a ship in the three days before Shabbos due to Bitul Oneg Shabbos

Ba'al ha'Ma'or (7a): It is like the Isur to besiege a city in the three days before Shabbos (taught right afterwards). It is like stipulating to desecrate Shabbos. The same applies to any danger that will necessitate Chilul Shabbos.

Rif (Shabbos 53a): We may bathe a baby before or after the Milah, whether the water was heated up on or before Shabbos.

Rebuttal (Ba'al ha'Ma'or): We may not heat up water on Shabbos before the Milah! If the hot water spilled, we postpone the Milah, like it says in Eruvin! We may heat up water only if it spilled after the Milah.

Ran (DH v'Heicha): The Ramban (134b DH Aval) permits Milah even if there is no hot water (to bathe the baby afterwards). After the Bris, we may heat water for Piku'ach Nefesh. He explains that in Eruvin, the water spilled before the Bris. After the Bris, even a Yisrael may heat it! The Ba'al ha'Ma'or says that the water spilled after the Bris. Had it spilled before, the Milah would be postponed. The Rashba proved so from Beitzah 2a: one may slaughter a bird on Yom Tov only if dirt was prepared to cover the blood. This is not a proof. Slaughter on Yom Tov is not a Mitzvah like Milah!

Ran (Shevuos 10a DH Malkin): If one swore not to eat for seven days, he (should) wait until he is close to mortal danger, and then eat enough to survive. The Rambam says that we lash him and he may eat immediately. I agree, for he swore to be Mevatel the Mitzvah to guard one's life

Tosfos (Shabbos 45a DH mi'Kamei): At a time of danger, one puts Ner Chanukah on his table (21b)! Rav discusses one who improperly put it outside.

Tosfos (Eruvin 100a DH Matan): If bloods of different Korbanos were mixed, the Aseh to throw the blood on the Mizbe'ach does not override the Lav of throwing more than required, for the need to do so came through negligence.

Poskim

Shulchan Aruch (OC 248:4): One may not leave with a caravan to the Midbar within three days before Shabbos if they will need to desecrate Shabbos due to danger. They may leave on Sunday, Monday or Tuesday. If later they must desecrate Shabbos due to danger, it is permitted.

Beis Yosef (DH v'Chosav Od, citing Rivash 17): Three days after Shabbos pertain to the previous Shabbos (Gitin 77a). Three days before Shabbos pertain to the coming Shabbos, therefore one may not put himself in danger that will necessitate Chilul Shabbos.

Ri ben Lev (2:53): All forbid if he will surely need to desecrate Shabbos.

Magen Avraham (14): People rely on this nowadays to put themselves in danger and not be Mechalel Shabbos.

Igros Moshe (OC 1:127): This does not show that we may not be Mechalel Shabbos to save one who endangered himself. No one says so. The Ba'al ha'Ma'or forbids after Tuesday, for it is like stipulating to desecrate Shabbos. If there were no Heter for Piku'ach Nefesh in such a case, we would permit embarking, but without a Heter for Piku'ach Nefesh! All (except for perhaps the Rif) agree that the Torah permits in any case. If even one who left on Sunday had no Heter, how would the Ba'al ha'Ma'or explain why we permit until Tuesday?! No one argues with the Ba'al ha'Ma'or (all permit Piku'ach Nefesh for one who endangered himself). Chasidim Rishonim had Bi'ah only on Wednesday (or later in the week), to avoid Chilul Shabbos due to birth on Shabbos (like Shmuel, that birth is exactly 271, 272 or 273 days from Bi'ah.) If Piku'ach Nefesh were forbidden for one who endangered himself, we should forbid Bi'ah before Wednesday, the source of the danger! Also, we should say that Chasidim refrained due to danger, not due to Chilul Shabbos! Rather, all agree with the Stam Mishnah that allows Chilul Shabbos for birth, even though she endangered herself by having Bi'ah before Wednesday. Piku'ach Nefesh is permitted in any case. Chasidim Rishonim were stringent.

Igros Moshe (DH u'Mah): Surely, Chachamim would not forbid Piku'ach Nefesh when one endangered himself, since the Torah permits it. Perhaps they have no such power! Ri ben Lev forbids embarking if he will Vadai need to be Mechalel Shabbos. He forbids after Wednesday if there is a Safek. He agrees that b'Di'eved, Piku'ach Nefesh is permitted. The Magen Avraham does not mean that people endanger themselves when there is Safek danger. One who does so is liable for his own death! Rather, if only some people consider a matter to be something dangerous, e.g. to be alone, we may be Mechalel (only) for them, for we cannot guarantee that it is not. One who embarks shows that he does not consider it to be dangerous.

Mishnah Berurah (27): One may not stipulate with the caravan that he will be Mechalel Shabbos if needed, for this is like stipulating to desecrate Shabbos.

Shulchan Aruch (OC 329:1): All Piku'ach Nefesh overrides Shabbos.

Chachmas Shlomo: An Aseh is not Docheh a Lav if it is due to his Peshi'ah (negligence). If one put himself into danger, Piku'ach Nefesh does not override Shabbos or Lavim. In Shabbos 45a, Rashi explains that the Sha'as ha'Dechak was danger. Why did Rav say that we rely on R. Shimon in Sha'as ha'Dechak? All permit Piku'ach Nefesh! Rather, the danger was due to his Peshi'ah (at a time of danger, one should place Ner Chanukah on his table, or not light at all), therefore it does not override Lavim.

Rebuttal (Tosefes Shabbos (280) and Machatzis ha'Shekel 279:1): It was not a true (mortal) danger, just close to danger.

Defense (Chachmas Shlomo): Tosfos' question shows that he disagrees.

Support (Be'er David 7 DH v'Heveisi): The Ran says that if one swore not to eat for seven days, when he is close to mortal danger, he eats and we lash him. Why is he lashed? Since his Peshi'ah caused the danger, Piku'ach Nefesh does not (fully) permit his oath. Likewise, one who brought himself to danger and needed to be Mechalel Shabbos must do Teshuvah for this. Tosfos (Eruvin 100a) supports this.

Bach (OC 671 DH uv'Sha'as): The Ran and Shulchan Aruch hold that there was a decree of Shmad (not to observe Mitzvos). If not, Yisre'elim would not endanger themselves for Ner Chanukah.

Question (R. Akiva Eiger): The Ran and Rema require Mesiras Nefesh only for Lavim, even at a time of Shmad!

Divrei Yatziv (OC 167:1): One may not learn from Sanhedrin 72b a Heter for Piku'ach Nefesh for one who endangered himself. We unearth him when Yesh Lo Damim (e.g. a father), for he did not endanger himself. We mourn over one killed by the kingdom for theft. He is not like one who committed suicide, for he thought that he can escape (Shach YD 345:5).

Note: Seemingly, even a father endangered himself. If one does not know that he is the father, he may assume that the thief plans to kill!