Nedarim, Chapter 9, Mishnah 1. This mishnah introduces the subject of releasing a vow by means of a 'pesach' (an opening). 'Pesach' means an opening: the sage who releases the vow, or the court, turns to the one who made the vow and asks him: "Had you known of this or that factor at the time you made your vow, would you have vowed?" And he answers that he did not know of it.
An example to illustrate the matter:
A man vowed to go to the mikvah every day, and he did not know that on Tuesdays the water of the mikvah is cold. They say to him: "Had you known that the mikvah is cold on Tuesdays, would you have vowed to go every day?" And he answers: "No." It turns out that the vow was made on a mistaken basis, and therefore the vow is released.
The question the mishnah raises:
Can a person always be believed that the opening found for him is genuine? There are cases in which a person is liable to feel ashamed and not tell the truth, and even to lie: when he is asked, "Would you have vowed had you known such and such?", he answers, "I would not have vowed," when in fact he would have vowed in any case.
The wording of the mishnah:
"Rabbi Eliezer omer: potchin l'adam bichvod aviv v'immo" - a vow may be released through an opening that relates to a person's honoring of his father and mother. The one releasing the vow asks him: "Would you have made this vow had you known that it involves an affront to the honor of your father and mother, and that people will come to demean your parents because you are in the habit of making vows, which is considered something negative?" And he answers: "No, I would not have vowed." In Rabbi Eliezer's opinion, he is believed.
"vachachamim osrin" - this opening may not be used, since a person is embarrassed to say that he would have vowed in any case and that he did not care about his parents' honor. Out of embarrassment he says, "I would not have vowed," when in reality he would have vowed, and therefore this is not a valid opening.
"Amar Rabbi Tzadok: ad shepotchin lo bichvod aviv v'immo, yiftechu lo bichvod haMakom" - instead of providing him with an opening based on the honor of his father and mother, provide him with an opening based on the honor of the Holy One, blessed is He: would you have vowed had you known that with these vows you are about to disgrace the Holy One, blessed is He? And if such an opening were valid, "ein nedarim" - there would be no vows at all. The Gemara explains that the meaning is that vows would not be properly released, because people do not tell the truth: everyone will say that he would not have disgraced the Holy One, blessed is He, and this is certainly not the truth, since perhaps he would have been willing to vow in any case.
The case in which the Sages agree with Rabbi Eliezer:
Even though the Sages and Rabbi Eliezer disagree about whether an opening may be provided through the honor of one's father and mother, the Sages agree that this opening may be used in the following circumstances: if he made a vow in a matter between himself and his parents - that is, he has already revealed that he is prepared to treat their honor with disregard - he is believed when he says: "Had I understood how much disregard for my parents' honor is involved in this, I would not have vowed." For we see before us a person who is not afraid to tell the truth, that he did not care about his parents' honor, since he was prepared to vow against them. And since he was prepared to do so, we know that he is telling us the truth when he retracts.
In summary: in this mishnah we learned the principle of the 'pesach' in releasing vows - the discovery that the vow was made on a mistaken basis - and the question of whether the one who vowed can be believed. Rabbi Eliezer permits providing an opening through the honor of one's father and mother; the Sages forbid it out of concern that the one who vowed is embarrassed to tell the truth; and Rabbi Tzadok adds that if an opening is provided through the honor of parents, an opening should also be provided through the honor of the Almighty, and then "ein nedarim" - there would be no vows at all. And we learned that in a case where he made a vow in a matter between himself and his parents, even the Sages agree that he is believed.