Gemara

(Mishnah): If one accepted Nezirus, and then found that an animal (that he intended to offer when he shaves) was stolen, he is a Nazir only if it was stolen after his vow.

Nachum ha'Midi erred in this law when Nezirim ascended from Bavel and heard that the second Mikdash had been destroyed.

Nachum: Had you known that the Mikdash would be destroyed, would you have vowed?

They said 'no', and he annulled their Neziriyos.

Chachamim: Anyone who vowed before the Churban is a Nazir. One who vowed after the Churban is not a Nazir.

Nedarim 64a (Mishnah - R. Eliezer): We are Pose'ach using Nolad (subsequent unexpected developments);

Chachamim forbid.

Nazir 32b (Rava): (Here, R. Eliezer did not argue with Chachamim who refuted Nachum, and hold that we are not Pose'ach with Nolad. This is because) Chachamim overwhelmed R. Eliezer with proofs, and made him agree with them:

(Rava): Chachamim say that we are not Pose'ach with Nolad, but we are Pose'ach with conditional Nolad. We say 'when you vowed, had people told you about the Churban, would you have vowed?'

(Rav Yosef): Had I been there at the time, I would have told Chachamim that the second Churban was not Nolad! "Heichal Hash-m, Heichal Hash-m, Heichal Hash-m" alludes to (a third Mikdash after) the Churban of two Mikdashos.

Support (Abaye): They even knew when the second Churban would be - "Seventy seven-year cycles have been decreed on Your people and Your Holy city." (It will be 490 years after the first Churban.)

Rebuttal: Granted, the year was known, but the day of the year was not.

Nedarim 64b (Rav Chisda): R. Eliezer learns from Moshe. (He had sworn to Yisro that he would not leave him, for he feared Dasan 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.

65a (Mishnah - R. Meir): There are cases that resemble Nolad, but are not really Nolad. E.g. Levi vowed 'I will not marry Plonis because her father is evil'; and they told him that her father died or repented. He vowed 'I will not enter this house, for there is a vicious dog or a snake inside', and they told him that it died.

Chachamim disagree (they are truly Nolad. Some texts say 'Chachamim agree.')

Question: Death is (genuine) Nolad!

Answer #1 (Rav Huna): It is as if Levi made his vow contingent (on the father being alive).

Answer #2 (R. Yochanan): The case is, he already died before Levi vowed.

Objection (against R. Yochanan - R. Aba): The next Mishnah discusses a mistaken vow. Why do we need two Mishnayos teaching the same matter?

This is left difficult.

Rishonim

Rif and Rosh (Nedarim 22a and 9:3): If one vowed 'I will not marry Plonis because her father is evil'; it is as if he made his vow contingent (on the father being alive and a Rasha). The Yerushalmi says that (if her father died or repented), he need not ask a Chacham to permit it. R. Yochanan was refuted.

Gra (YD 232:55): Except for the Rashbam, the Poskim say that Kashya (this is left difficult) is not a refutation. Elsewhere, the Rif and Rosh themselves say so! In any case, the Halachah follows Rav Huna because the Yerushalmi supports him.

Rosh: The Gemara favors the text that says 'Chacham agree with R. Meir.'

Rambam (Hilchos Shevuos 6:1): If one swore and regrets it and sees that fulfilling it will cause pain, he asks a Chacham to permit it. The same applies if he regrets it because a new matter arose that he did not think about when he swore.

Rambam (Hilchos Nedarim 8:2): If Levi vowed 'I will not marry Plonis because her father is evil', or 'I will not enter this house, for there is a vicious dog inside', and the father or dog died or her father repented, he is permitted. It is as if he vowed not to marry her or enter the house until the damage is removed.

Rosh (9:1): The Yerushalmi says that becoming regret is not Nolad.

Perush ha'Rosh (Nazir 32b DH Aval): We say 'had people counseled you at the time not to accept Nezirus, lest there be a Churban before your Nezirus finishes, would you have vowed?'

Poskim

Shulchan Aruch (228:13): Even though we are not Pose'ach with Nolad, we are Pose'ach with Tanai Nolad. E.g. if one accepted Nezirus, and then the Mikdash was destroyed, we are not Pose'ach to say 'had you known that the Mikdash would be destroyed, would you have vowed?' Rather, we are Pose'ach 'had someone lied to you at the time and told you that the Mikdash was destroyed, would you have vowed?' and similar matters.

Prishah (21): Sometimes we cannot be Pose'ach with Tanai Nolad. E.g., if he was in Yerushalayim, he would know that the Mikdash was not destroyed.

Question (Beis Yosef): Why did the Rambam omit this? Rava says so (Nazir 32b), and no one disagrees!

Answer (Beis Meir, cited in Hagahos Tur ha'Shalem 126): Perhaps the Rambam holds like Perush ha'Rosh, that we say 'had people told you ... perhaps there will be a Churban...' This Pesach is included in 'had 10 people appeased you at the time...' In that case, the Rosh requires that now 10 people appease him. Perhaps the same applies here.

Rema: We are not Pose'ach with Nolad to make a Pesach out of Nolad in order that he will regret, for surely he regrets only from the Nolad and onwards. If he regrets from the beginning, even if it is due to the Nolad, this is called regret and we permit him.

Shach (24): This is because we permit even without a Pesach, as long as there is regret from the beginning (Sa'if 7). This is why nowadays we are Pose'ach even with Nolad, but we must say 'had you known that you will regret it, would you have vowed?' (Rema, Sa'if 7)

Shulchan Aruch (232:19): If one specified when he vowed why he vows, it is as if he made his vow contingent on the matter. E.g. if he said 'I will not marry Plonis because her father is evil', and they heard that her father died or repented, he need not permit the vow. It is as if he made a Tanai. Since the Tanai is Batel, his Neder is Batel.

Question (Prishah 15): Why did the Tur (and Shulchan Aruch) teach a Neder 'because my wife stole his wallet' amidst Nidrei Shegagos (Sa'if 6)? It should be included amidst making a Neder contingent (Sa'if 19)!

Answer (Taz 32): There, the Neder was a mistake. Here, it was a true Neder, and afterwards it became Batel.

Shach (45): If one vowed not to marry Plonis because she is ugly, and she became pretty, he is (still) forbidden (Sa'if 6). There is different, because it is not common for this to happen, so he did not intend 'as long as she is ugly.' It is common for people to repent or die.