Gemara

(Mishnah - R. Eliezer): We are Pose'ach with Nolad (unexpected developments). E.g., Reuven vowed not to benefit from Ploni, and Ploni became the town scribe, or Ploni was about to marry off his son (and Reuven wants to attend);

Reuven says, had I known that he would become the scribe, or would marry off his son in the near future, I would not have vowed.

Or, one said 'I may not enter this house', and it became a synagogue;

Had he known that it would become a synagogue, he would not have vowed;

Chachamim forbid such cases.

Nedarim 64b (Rav Chisda): R. Eliezer learns from Moshe. (He had sworn to Yisro that he would not leave him, for he feared Dosan and Aviram. Hash-m told Moshe that he can annul his vow,) "for all the men have died..." Death is Nolad!

Chachamim argue because they hold that they did not really die:

(Reish Lakish): (Hash-m said that they died because) they became poor.

23a: R. Yishmael bar R. Yosi came before Chachamim to permit a vow. Whatever they suggested that he might have overlooked, he had it in mind when he vowed. A launderer hit R. Yishmael with his rack for paining Chachamim.

R. Yishmael permitted his own vow, for he did not vow with intent to be hit!

Question (Rav Acha mi'Difti): This is Nolad. He did not expect to be hit. The Halachah is, we are not Pose'ach with Nolad!

Answer (Rav Ashi): It is not Nolad. Many wanton people pain Chachamim.

Kesuvos 62b: R. Akiva was Mekadesh Ben Kalba Savu'a's daughter. Her father vowed that she would not benefit from his property (because R. Akiva was unlearned at the time). R. Akiva became a great Chacham came. Ben Kalba Savu'a sought to annul his vow.

R. Akiva: When you vowed, did you intend to forbid her even if her husband would become a great Chacham?

Ben Kalba Savu'a: Had I known that he would learn even one Perek, or even one Halachah, I would not have vowed.

R. Akiva revealed himself; his father-in-law gave half his money to him.

Rishonim

The Rif and Rosh (9:1) bring the Mishnayos in Nedarim 64a and the Gemara, and the episode with R. Yishmael b'Rebbi Yosi.

Rosh: The Yerushalmi says that becoming poor and regret are not Nolad.

Rambam (Hilchos Shevu'os 6:12): We do not permit vows using Nolad. E.g., Reuven vowed not to benefit from Ploni, and Ploni became the scribe of the town. Since he does not regret the oath, we do not use this for a Pesach. Even if he himself says 'had I known this, I would not have sworn', we do not permit him, because he does not regret. He does not want to benefit from Ploni; he wants someone else to be the Sofer. However, if he himself regretted due to the Nolad and his mind changed, we permit him. The same applies to all similar cases.

Kesef Mishneh: The Rambam says this to explain why R. Akiva was able to permit Ben Kalba Savu'a's vow. Ben Kalba Savu'a wanted R. Akiva to become a Chacham! However, if so what was R. Eliezer's proof from Moshe? Moshe wanted them to die! Perhaps R. Eliezer proves that there is a Heter for some case of Nolad, and we have no source to distinguish between different forms of Nolad.

Ra'avad: We are Pose'ach with a common Nolad, like the case of R. Yishmael b'Rebbi Yosi. Hash-m was Pesach for Moshe because poverty is more common than death. The distinction between common and uncommon is only when he regrets only due to the Nolad. If he says 'I vowed due to anger or sadness, and now that it passed I regret', we permit in any case. What the Rambam wrote is unclear.

Kesef Mishneh: The Rambam does not distinguish common and uncommon Nolad. He explains the Gemara's answer (that Dosan and Aviram only became poor,) like Rashi, that they were already poor when Moshe vowed.

Poskim

Shulchan Aruch (YD 228:12): We are Pose'ach only with a very common matter, but not with something not so common. E.g., Reuven vowed not to benefit from Ploni or enter his house, and Ploni became a Chacham whom everyone needs or his house became a Beis ha'Keneses. We are not Pose'ach 'had you known that he would become a Chacham or his house would became a Beis ha'Keneses, would you have vowed?' Through a Pesach the vow is uprooted retroactively, but this Pesach does not uproot it. Even had he considered the possibility, he would not have refrained from vowing, since it is not common.

Question: If when there is a common Pesach we consider the vow to be a mistake, why must he ask a Chacham? Nidrei Shegagos are among the four vows, which are permitted without asking a Chacham!

Answer (Beis Yosef DH v'Im, citing Ramban (really, Rashba Teshuvah 273)): It is like a mistaken vow, but it is not truly a mistake. Nidrei Shegagos are when there is an absolute mistake, e.g. he thinks that his wife hit his son or stole his wallet. Common Nolad is like partial Ones. It is not a full Ones.

SMaG (Lav 241 70b, brought in Beis Yosef DH u'SMaG): Nedarim 23a shows that we are Pose'ach with common Nolad. If he regrets from the beginning, we are not concerned if this is due to Nolad. R. Asi permitted by asking only if the Noder regrets, without asking whether Nolad caused his regret. The Re'em agrees.

Beis Yosef (228:7, DH veha'Mordechai): Rav Yosef Bechor Shor says that there are three regrets. All permit with something that already occurred, and had the Noder known he would not have vowed. Nolad, i.e. something totally unexpected, cannot be used. Even if the Noder considered that it might occur, he would not have refrained. Amora'im argue about regret due to a common development after the Neder. The stringent opinion considers this like Nolad.

Beis Yosef (DH Kasuv): The Ramban (ibid.) is not Pose'ach with Nolad, but we are Pose'ach with regret due to Nolad. This is unlike Rav Yosef Bechor Shor (above).

Taz (17): Tosfos is difficult. It seems that Ben Kalba Savu'a did not know that R. Akiva had gone to learn (so his vow was mistaken). The Beis Yosef explains that we are not Pose'ach with Nolad, i.e. we do not suggest that he regrets (from the beginning), but if he himself says so, it helps. I say that Rav Yosef Bechor Shor agrees. Even if he does not, the Rema (Sa'if 13) rules like this because the Rambam agrees with the Ramban.

Beis Yosef (DH v'Nireh): The Re'em says that we are not Pose'ach when regret is from the Nolad and onwards. SMaG understands that this means that he did not want the Nolad to occur. It cannot mean that he does not regret from the beginning, for this never suffices (even without Nolad).

Rema: Also death is not common. Poverty is common. Pregnancy is common. It is not called Nolad.

Beis Yosef (DH u'Piresh): Tosfos (Kesuvos 63a DH a'Da'ata) says that R. Akiva was able to permit Ben Kalba Savu'a's vow because it is common that one who goes to learn becomes a great Chacham. However, it must be very common. Why isn't death common enough? It happens every day! We must say that death of one's enemies is not common. Alternatively, it is just as likely that the enemy will die after the Noder as before, so it is not considered common. The Ramban (ibid.) says that death is not considered common even if the person is sick, for most sick people recover.

Shach (22): This is even if he was very sick. If he was Goses it is not Nolad, for most Gosesim die.