Shevuos, Chapter 3, Mishnah 9. This mishnah deals with someone who swore two oaths that contradict one another.
The case:
"Shevuah she'ochal kikar zo" - "I swear that I will eat this loaf." This is a shevuas bitui, an oath of expression, which places upon him an obligation to fulfill what he swore to do and eat the loaf.
"Shevuah shelo ochlenah" - "I swear that I will not eat it." This second oath, which contradicts the first, is a shevuas shav, a vain oath. As we learned in the previous mishnah, one who swears to transgress the Torah - his oath is a vain oath. And here, once he first swore to eat the loaf, the Torah obligates him by force of his oath to eat it, and now he is swearing not to do so. This vain oath takes effect at once, the moment it is spoken.
Two situations following the oaths:
"Achalah" - he ate the loaf about which he swore in the first oath. He transgressed only the second oath, the vain oath, since a vain oath applies from the moment it is spoken, independent of what he does afterward. As for the first oath, he did not transgress it at all, since he did exactly what he swore to do.
"Lo achalah" - he refrained from eating the loaf and thereby contradicted his first oath. In this case he transgressed both oaths: the vain oath, for which he became liable the moment he spoke it, and also the oath of expression, since he swore to eat the loaf and did not eat it.
In summary: In this mishnah we learned the law of two contradictory oaths - "Shevuah she'ochal kikar zo" followed by "Shevuah shelo ochlenah." The first is an oath of expression and the second is a vain oath, which takes effect immediately upon being spoken. If he ate the loaf - he transgressed the vain oath alone; if he did not eat it - he transgressed both the vain oath and the oath of expression together.