"If only I would have known.." After many pages delineating the parameters of various vows the Talmud discusses when and how one may annul a vow. "Heter nedarim porchin b'aveer, the annulments of vows are floating in wind and they have nothing to rely on" (Chagigah 10a). Nonetheless our Sages outlined parameters in which a vow may be annulled. We might call it giving someone a second chance; people often say things they wish they had not and under certain conditions we can ignore what was said. The details of those conditions is the theme of the ninth chapter of Masechet Nedarim.
Annulments generally are based on the fact that one is sorry he vowed, having not taken into account the ramifications of one's vow or circumstances change to the extent that had such existed at the time of the vow one would not have made the vow. The ninth chapter begins with a debate regarding the former method.
"Rabbi Eliezer says one may use as an opening [to annul a vow] the honour of one's mother and father; the Sages forbid [this]" (Nedarim 64a). Our Sages looked askance at those who took vows; the need to do so often demonstrates a weakness of character that one would need a vow to do what is right or avoid what is wrong. Many vows served to forbid the permissible something viewed with disfavor - as observing the laws of the Torah should be enough without adding on extra prohibitions[1]. There was also fear that one would violate a vow, a serious prohibition. For these reasons it was better to avoid vows.
Rabbi Eliezer thus argues that the court in attempting to annul a vow may say "had you realized that people would say regarding your parents, 'woe unto the father who has raised a wicked son[2] like this who is lax in vows' (Rashi 64 a s.v. Rabbi Eliezer omer) would you have taken such a vow?" If and when he answers that he would not, the court may annul the vow.
The Sages disagree arguing that this answer can't be taken seriously and may in fact be a lie. What child would actually say "yes I would have taken that vow despite the embarrassment it might have caused my parents". Perhaps the child really does not care how his vow would impact on his parents - this would not be the first, or last, time a child would do so. If so, argue the Sages this cannot serve as the basis for annulling a vow.
More telling is that Rabbi Eliezer is not even worried the child will lie. A child will have no compunction about saying yes I would have taken such a vow despite the pain it may have caused my parents.
Jewish law (thankfully) accepts the view of the Sages[3] a view Rabbi Eliezer shares regarding the honour of G-d. Thus we cannot annul a vow by asking "had you realized that taking vows goes against the wishes of G-d would you have done so?" People would not (at least in Talmudic times) actually say they would have - despite the fact that they may very well do so. Thus their answer of "of course not" cannot be grounds for nullification.
What is fascinating about this discussion is that there is no actual Torah prohibition to take a vow. It may be frowned upon but it does not actually violate Jewish law.
Technical observance of mitzvoth is not the definition of doing the "straight and the good in the eyes of G-d" (Devarim 6:18). There is so much more to being a good person and good Jew than technical mitzvah observance. As the Ramban, in perhaps his most well-known comment, notes (Vayikra 19:2 and see the Ramban on Devarim 6:18) one can be a disgusting person all the while following the mitzvoth to the letter. We must act lifnim mesurat hadin[4] beyond, or more accurately in front, of the laws of the Torah, acting in ways to ensure that the mitzvoth sanctify us.
While one will not knowingly say that one would ignore the desire of G-d, almost paradoxically the Mishna rules that this is only so in regard to a general question of acting against the honour of G-d. But when it comes to specific mitzvoth people are more than willing to say that yes, even had they known they were violating a mitzvah they would have done so nonetheless. Thus the Mishna (Nedarim 65b) quotes Rav Meir - and no dissenting view - that we can nullify a vow with an affirmative answer to "had you known you would [with your vow] have violated 'do not take revenge or bear a grudge, do not hate your brother in your heart, love your neighbour as yourself, your brother shall live with you' would you have done so?" Because he might say he still would have taken such a vow we can believe him when he says he would not have.
We are often so convinced that we must act as we do even if such "seems" to go against the Torah, arguing that in this case violating this mitzva is in fact honouring G-d. We may not quite articulate it that way but the human capacity for justification is great.
Note how all these are examples of mitzvoth between man and man. Oftentimes our zeal for mitzvoth between man and man is often much less than between man and G-d.
[1] While frowned upon taking a vow would not violate the prohibition of adding to the Torah. Such is violated only when one adds to an existing mitzva i.e. if one were to lift five species instead of four on Sukkot, or if one claims that some practice is a biblical command when in fact it is not.
[2] While to the modern ear the term rasha, evil one, is a most harsh term it may not always have quite such a negative a connotation in rabbinic literature. It is worth noting that the Rambam (Laws of Teshuva 3:1) defines as a rasha one who has 50% plus 1 aveirah, sin vs. 50% less one mitzvah. There clearly are levels of evil.
[3]We have a similar ruling regarding the mitzva of kivud av veim. While one need not heed one's parents wishes regarding a profession to enter one may not enter a profession if such would legitimately cause the parents ridicule or embarrassment.
[4] The obligation to act beyond the letter of the law - almost by definition an oxymoron - is derived from the verse in Devarim noted above. It is obvious but needs to be stressed the Torah contains so much more than a listing of the 613 mitzvoth.
