Nedarim, Chapter 9, Mishnah 8. The Mishnah deals with a person who made a vow and gave a reason for it, and afterward it became clear to him that the reason does not hold true in all cases.
One who vows against wine:
A person says "Konam yayin she'eini to'em" - he vows that he will not taste wine, and he explains his vow: "shehayayin ra lame'ayim" - because wine is bad for the intestines.
They said to him: "vahalo hameyushan yafeh lame'ayim" - but aged wine is good for the intestines. 'Aged' wine, as the Gemara in Bava Basra explains, is wine that is three years old ('old' wine is from the previous year), and such wine is good for the intestines and beneficial for the stomach. The Mishnah rules: "hutar bameyushan" - the aged wine became permitted to him. And not only that, but "velo bameyushan bilvad hutar, ela bechol hayayin" - it is not only the aged wine that became permitted to him, but all wine.
When is all wine permitted?
This sweeping leniency applies only when his response to the new information takes one of two forms:
"Had I known this, I would not have made this vow at all."
"Had I known this, I would have said that old wine is permitted, and only new wine is forbidden to me."
But if he said: had I known that aged wine is good for the intestines, I would have said that all wine is forbidden to me except for it - then only the aged wine became permitted to him, and all the other wines remain forbidden.
One who vows against onions:
Another example: one who says "Konam batzal she'eini to'em" - onion is konam, so that I will not taste it, and he explains: "shehabatzal ra lalev" - because onion is bad for the heart. They said to him: "vahalo hakufri yafeh lalev" - but the village onion is good for the heart. There is a type of onion that comes from the villages, and it is good for the heart and beneficial to it. Therefore: "hutar bakufri" - the village onion became permitted to him, "velo bakufri bilvad hutar, ela bechol habetzalim" - and not only the village onion became permitted to him, but all onions.
Here too, the leniency extends to all onions only on condition that he said, once the matter became known to him, that he would not have vowed at all, or that he would have said that the village onion is permitted and only an onion that is not a village onion is forbidden.
An incident that occurred:
The Mishnah concludes with "ma'aseh shehayah" - an incident that occurred involving onions, and its central figure is Rabbi Meir: a man forbade onions upon himself by a vow, for the reason that they are bad for the heart, and when it became clear to him that the village onion is good for the heart - "hitir lo Rabbi Meir bechol habetzalim" - Rabbi Meir permitted him all onions, so that he could eat any type of onion.
The reason for this: Rabbi Meir follows the approach of Rabbi Akiva, which we already learned in the previous Mishnah, that a vow which is partially released is entirely released.