Nedarim, Chapter 9, Mishnah 10. This mishnah opens a series of cases in which the vow was mistaken from the outset, and therefore does not even require formal release.
The text of the mishnah:
"Konam she'eini nosei es plonis ke'urah, venimtzeis na'ah; shechorah, venimtzeis levanah; ketzarah, venimtzeis arukah - mutar bah" - a man vows not to marry a particular woman, and gives as his reason a trait he thought he saw in her:
"ke'urah, venimtzeis na'ah" - he vowed because he assumed she was unattractive, and it turned out that she is not unattractive at all, but beautiful.
"shechorah, venimtzeis levanah" - he assumed she was dark, and it turned out that she is fair.
"ketzarah, venimtzeis arukah" - he assumed she was short, and it turned out that she is tall.
In all of these cases he is permitted to her. The mishnah stresses the reason: "lo mipnei shehi ke'urah vena'aseis na'ah, shechorah vena'aseis levanah, ketzarah vena'aseis arukah, ela shehaneder ta'us" - the permission does not stem from some change that came over her, that she was unattractive and became beautiful, dark and became fair, short and became tall; rather, it is because the vow was mistaken from the very start. And since it was a mistake, not even a formal release of the vow is needed.
The gemara's addition - the view of Rabbi Yishmael:
At this point a passage is missing from the wording of the mishnah, and the gemara notes that the mishnah is incomplete here and must be supplemented. According to that addition, the mishnah should read: "Rabbi Yishmael omer: afilu ke'urah vena'aseis na'ah, shechorah vena'aseis levanah, ketzarah vena'aseis arukah - mutar bah" - that is, even in a case where a change actually did come over her: she was unattractive and became beautiful, dark and became fair, short and became tall - even in such a case he is permitted to her.
In the cases Rabbi Yishmael speaks of, however, a formal release of the vow will be required, since here the vow was not mistaken from the outset. It is simply that the change counts as a valid opening through which to release the vow, and it is not considered a 'nolad' - that category of opening with which one may not open a vow, which we learned about earlier in the chapter.
The incident with his sister's daughter:
The mishnah relates an incident: "uma'aseh be'echad shenadar mibas achoso hana'ah" - a man vowed not to derive benefit from his sister's daughter, after people pressed him to marry her, since there is a special value in marrying one's sister's daughter. He made his vow so that they would stop pestering him about it.
"vehichnisuha leveis Rabbi Yishmael veyipuha" - they brought her into Rabbi Yishmael's house and beautified her, doing everything necessary to make her appear more attractive. Afterward, "amar lo Rabbi Yishmael: beni, lazu nadarta?" - was it about this woman that you vowed, is this the woman you said you did not want to marry? "amar lo: lav" - she looks entirely different. "vehitirah Rabbi Yishmael" - Rabbi Yishmael released him from his vow on the basis of that opening.
"be'osah sha'ah bacha Rabbi Yishmael ve'amar: benos Yisrael na'os hen, ela she'aniyus menavaltan" - the daughters of Israel are beautiful in themselves, and it is poverty that disfigures them, for they lack anyone to provide them with the care and cosmetics they need.
And because Rabbi Yishmael cared so deeply for the daughters of Israel, when he passed away the daughters of Israel would lament him, saying: "Daughters of Israel, weep for Rabbi Yishmael" - echoing the words of the prophet about King Shaul: "Daughters of Israel, weep over Shaul." Just as they wept over Shaul's death, so did they weep over the death of Rabbi Yishmael, because of the concern he showed for the daughters of Israel.
In summary: in this mishnah we learned that a vow made on the basis of a mistake is void on its own and requires no release at all, as emerges from the three cases: he thought she was unattractive and she turned out to be beautiful, dark and she turned out to be fair, short and she turned out to be tall. We also learned the gemara's addition presenting Rabbi Yishmael's view, that even when a change actually did come over her he is permitted to her - though in that case a release is required, and the change serves as a valid opening that is not in the category of 'nolad'. And we concluded with the incident of his sister's daughter, with Rabbi Yishmael's weeping over the fact that "the daughters of Israel are beautiful, but poverty disfigures them," and with the lament of the daughters of Israel over him at his death.