Nedarim, Chapter 9, Mishnah 3. Rabbi Meir says: "Yesh devarim she'hen ke'nolad ve'einan ke'nolad" - there are matters that appear to be a nolad (a newly developed circumstance), and although with regard to an actual nolad Rabbi Meir holds like the Sages of the previous Mishnah that we do not open with it, nevertheless these particular matters do not carry the status of a nolad, and they may be used as an opening to dissolve the vow. However, as the Mishnah will conclude, "ve'ein chachamim modim lo" - the Sages do not agree even with this position of Rabbi Meir.
Examples of matters that resemble a nolad but are not a nolad:
"Konam she'eini nosei es plonis she'aviha ra" - a person takes a vow forbidding himself to marry a certain woman, and he states that the reason is that her father is a wicked man. "Amru lo: mes, o she'asah teshuvah" - they informed him that the father had died, or that he had repented and become a good person.
"Konam bayis zeh she'eini nichnas, she'ha'kelev ra be'socho o she'ha'nachash be'socho" - a person forbids himself to enter a house, and he states that the reason is a vicious dog inside it, or a dangerous snake found there. "Amru lo: mes ha'kelev o she'nehrag ha'nachash".
Concerning all of these the Mishnah rules: "Harei hu ke'nolad ve'eino nolad" - they resemble a nolad, but halachically they do not have the status of a nolad.
The reason for this:
Since the one who vowed stated the reason for his vow at the time he made it, the vow becomes almost like a condition: he declared certain facts and established that they are the cause of his vow. Therefore, even though the death of the dog or the killing of the snake, and likewise the death of the father or his repentance, occurred after the vow was made, this is not considered a nolad - for this was something the one who vowed had in mind from the outset, and he himself clarified that this was the reason for his vow. Once that reason has fallen away, he can say: I would never have vowed had this circumstance not existed.
The position of the Sages:
Nevertheless the Mishnah concludes: "Ve'ein chachamim modim lo". The Sages disagree and hold that even these matters are considered a nolad, and they may not be used as an opening to dissolve the vow, since they did not exist at the time the vow was made.
In summary: In this Mishnah Rabbi Meir teaches that there are matters that appear to be a nolad but are not a nolad - namely, where the one who vowed stated his reason at the time of the vow, and afterwards that reason fell away. Such is the case of one who vows not to marry a woman because her father is wicked, and the father then dies or repents, and likewise one who vows not to enter a house because of the dog or the snake inside it, and the dog dies or the snake is killed. In Rabbi Meir's view this is regarded as though he made his vow dependent on that reason, and therefore we may open with it; the Sages, however, do not agree with him, and hold that it has the status of a nolad, since it did not exist at the time of the vow.