Shevuot 29A

ואכל תאנים והפריש קרבן, ואחר כך אכל ענבים לחודייהו ⁠– הויא להו ענבים חצי שיעור, ואחצי שיעור לא מיחייב קרבן. הכא נמי, כגון דאמר: ״שבועה שלא אוכל עשר״, וחזר ואמר: ״שבועה שלא אוכל תשע״, ואכל תשע והפריש קרבן, ואחר כך אכל עשירית; הויא לה עשירית חצי שיעור, ואחצי שיעור לא מיחייב.

and he unwittingly ate figs and set aside an offering for breaking the second oath, and afterward he unwittingly ate grapes alone, he is exempt from liability for breaking the first oath. The grapes are tantamount to a half-measure of his first oath, which was not to eat figs and grapes together, and one is not liable to bring an offering for a half-measure. Here also, where he said: On my oath I will not eat 10, and then said: On my oath I will not eat 9, and he ate 9 and set aside an offering for breaking his second oath, and later he ate the tenth, this tenth is tantamount to a half-measure of his first oath, and one is not liable to bring an offering for a half-measure.

מתני׳ איזו היא שבועת שוא? נשבע לשנות את הידוע לאדם ⁠– אמר על העמוד של אבן שהוא של זהב, ועל האיש שהוא אשה, ועל האשה שהיא איש;

Mishnah: Which oath is an oath taken in vain, mentioned in the previous Mishnah (27b)? It is when one takes an oath to deny that which is known to people to be true, For example: one says about a stone column that it is made of gold, or about a man that he is a woman, or about a woman that she is a man.

נשבע על דבר שאי אפשר לו ⁠– ״אם לא ראיתי גמל שפורח באויר״ ו״אם לא ראיתי נחש כקורת בית הבד״;

Another type of oath taken in vain is when one takes an oath about a matter that is impossible, e.g., if he says: If I did not see a camel flying through the air, or: If I did not see a snake as large as the beam of the olive press.

אמר לעדים: ״בואו והעידוני״, ״שבועה שלא נעידך״; נשבע לבטל את המצוה ⁠– שלא לעשות סוכה ושלא ליטול לולב ושלא להניח תפילין; זו היא שבועת שוא, שחייבין על זדונה מכות ועל שגגתה פטור.

In the case of one who said to witnesses: Come and testify for me, and they replied: On our oath we will not testify for you, that is an oath taken in vain, because it involves taking an oath to refrain from performing a mitzva. Other examples of this include an oath not to build a sukka, or not to take a lulav, or not to don tefillin. This type of oath is an oath taken in vain, for which one is liable to receive lashes if he takes the oath intentionally, and for which he is exempt if he takes it unwittingly.