Shevuos, chapter 3, mishnah 4. This mishnah deals with a person who took an oath not to eat, and with two special situations: when the food he ate is not fit for eating at all, and when it is a food that the Torah forbids to eat.
The first part of the mishnah - items that are not fit for eating:
"Shevuah shelo ochal, v'achal ochalim she'einam re'uyim la'achilah v'shasah mashkin she'einam re'uyim lishtiyah - patur" - a person took an oath that he would not eat, and afterward he ate things that are not meant for eating at all, or drank liquids that are not fit for drinking. He is exempt, since his oath was that he would not eat, and this is not considered eating or drinking at all.
The second part - items that are forbidden to eat:
"Shevuah shelo ochal, v'achal nevelos u'treifos, shekatzim u'remasim - chayav, v'Rabbi Shimon poter" - if he took an oath that he would not eat, and then ate forbidden foods: neveilah (an animal that was not slaughtered properly), treifah (an animal found to have a defect in its internal organs), or creeping and crawling creatures - all of these are forbidden to eat by Torah law. According to the Tanna Kamma he is liable for violating his oath, and Rabbi Shimon exempts him.
Rabbi Shimon's reasoning:
A person is already "sworn and standing from Mount Sinai" not to eat these things. The Torah's prohibition is itself like an oath, so an earlier oath already rests upon him - and an oath does not take effect upon an oath. Therefore, although he transgressed a Torah prohibition, he did not transgress the prohibition of an oath.
The view of the Tanna Kamma:
The Tanna Kamma also agrees with this principle that a person is sworn and standing from Mount Sinai. How, then, does he hold him liable? The Ran and the Rambam explain, based on the Gemara's conclusion, that the case is specifically where he included forbidden items together with permitted items in his oath - that is, he swore that he would not eat kosher food and non-kosher food as one.
Since he explicitly said that he would not eat kosher food, and the oath takes effect upon him, it takes effect as well upon the forbidden food included with it, even though a Torah oath already rests upon him not to eat it. This is what is called "including permitted items together with forbidden items," and in such a case the Tanna Kamma holds him liable. Rabbi Shimon rejects this and holds that even though he included kosher food along with it, the oath does not take effect upon the forbidden food.
The continuation of the mishnah - an act of eating that has already taken place:
The mishnah now discusses one who has already eaten things that are not kosher. Technically this is not a case of an oath but of a vow: a person forbids his wife to derive benefit from him using the word "konam," like a korban - that she should not be able to benefit from him just as she cannot benefit from a korban. And he makes it conditional: the vow will take effect only if he ate today, before he made the vow. And indeed he did eat that day, except that he ate food that is not kosher. Does that count as eating?
The Mishnah rules that his wife is forbidden to him, meaning that the vow takes effect and is binding. This is true even according to Rabbi Shimon, and here is the explanation:
The earlier dispute revolved around the question of whether a person intends to include in his oath, or is even able to include in it, matters that are already covered by an oath from Mount Sinai. Here, however, the discussion concerns an act that has actually taken place: he ate a forbidden food, and eating a forbidden food is considered eating in every respect. This is not like eating dirt, which is not fit for eating at all.
Therefore, when he says "im achalti" - "if I ate" - he did in fact eat, and no one disputes that this counts as eating. The whole question was only whether an oath can take effect where a prior oath already exists; but here the konam makes its validity depend on an act that occurred, and an act of eating did indeed take place, so the vow takes effect.
In summary: In this Mishnah we learned that one who swears not to eat and then eats things that are not fit for eating is exempt, since that is not considered eating. If he ate forbidden foods, the first Tanna holds him liable, while Rabbi Shimon exempts him because he is already under oath from Mount Sinai, and an oath does not take effect upon an existing oath. According to the Ran and the Rambam, the case is one where he included permitted items together with the forbidden ones. And at the end we saw that one who makes a konam conditional on having eaten today - his vow takes effect even if what he ate was a forbidden food, since such eating is a full act of eating.