1. נשבע לבטל את המצוה, machlokes about נשבע לקיים את המצוה

The next Mishnah states: נשבע לבטל את המצוה – if one swore to violate a mitzvah, and he did not, he is exempt from שבועת ביטוי liabilities. If one swore לקיים – to fulfill a mitzvah, and did not, the Tanna Kamma says he is exempt. Rebbe Yehudah ben Beseira says he is liable, based on a kal vachomer: if one is liable for swearing about רשות – an optional act, שאינו מושבע עליו מהר סיני – about which he is not sworn from Har Sinai to perform, then if one swears to fulfill a mitzvah, which he is already sworn to perform, he should certainly be liable!? The Rabbonon respond that an optional act is different, שכן עשה בה לאו כהן – because [the Torah] equated a negative [shevuah] with a positive one (i.e., his shevuah to do or not do that act is effective), but swearing to fulfill a mitzvah is not effective, since swearing to violate one is not. A lengthy discussion proves the passuk is not discussing shevuos about mitzvos.

Rebbe Yehudah ben Beseira would respond that a shevuah for הטבת אחרים – benefiting others is binding, even though swearing to harm them is not!? The Rabbonon would counter that its parallel negative shevuah is "לא איטיב" – “I will not benefit others,” which is valid.

1. The difference between שבועה שלא אוכל and שבועה שלא אוכלנה

The next Mishnah states that one who swears שבועה שלא אוכל ככר זו – “An oath that I will not eat this loaf,” שבועה שלא אוכלנה שבועה שלא אוכלנה – “an oath that I will not eat it, an oath that I will not eat it,” and then ate it, he is only liable once, because the additional shevuos cannot prohibit something already prohibited by a shevuah. The Gemara wonders why the Mishnah changes terminology from "שבועה שלא אוכל" to "שבועה שלא אוכלנה", and explains that it teaches that the second shevuah is only ineffective because he used this order. Had he sworn in the reverse order, the second shevuah would take effect, because Rava said that one who swears שבועה שלא אוכל ככר זו – “an oath that I will not eat this loaf” is liable as soon as he eats a כזית (the standard eating shiur), but one who swears שבועה שלא אוכלנה – “an oath that I will not eat it” is liable only for eating the entire loaf. Therefore, if he first swore “I will not eat it,” and then swore “I will not eat (the loaf),” the first only prohibits eating its entirety, and the second prohibits even a כזית, and is therefore effective.

1. אם נשאל על הראשונה עלתה לו שניה תחתיה

The Gemara asks why the Mishnah discussed one who made three successive shevuos, when the same law could be taught with two shevuos. It answers that the extra case teaches: חיובא הוא דליכא – there is no liability for the redundant shevuah, הא שבועה איכא – but the shevuah is there (and not void), דאי משכחת רווחא חיילא – so that if it later finds a space to take effect, it takes effect. Rava taught the practical application: שאם נשאל על הראשונה, עלתה לו שניה תחתיה – if he requests [a heter] for the first shevuah, the second takes effect on him in its stead. The Gemara suggests supporting this principle from a Baraisa teaching that if one accepted two terms of nezirus, and after counting the first term and designating the korban, annulled his first nezirus, the second nezirus is fulfilled through the completed first term. However, the Gemara responds that this case is different, since even if he had not annulled the first nezirus, he would have to observe the second term (afterwards), in contrast to a second shevuah, which has no practical impact.

Siman - Kazoo. At the kazoo parade, where kids blew their kazoos when the band leader made a shevuah לקיים את המצוה but did not fulfill it, one man spotted a cake float and made two back-to-back kazoo-blown oaths not to eat it—שלא אוכל עוגה זו שלא אוכלנה—only to learn that if he gets a היתר for the first שבועה, the second one still buzzes into effect.