Orlah, Chapter Two, Mishnah 15. This mishnah is almost identical to the previous one, which dealt with the combining of prohibitions, except that instead of leaven it deals with spices.
The text of the Mishnah:
"Tavlin shel Terumah v'shel kil'ei hakerem shenaflu l'toch hakedeirah" - spice of Terumah and spice of kil'ei hakerem (mixed crops of a vineyard) that fell into a pot of food that was cooking.
"Lo b'eilu k'dei l'tabel v'lo b'eilu k'dei l'tabel" - in each spice on its own, this one of Terumah and that one of kil'ei hakerem, there is not enough to create the taste of the spice, and it is impossible to taste either of them separately; only when the two are combined can they be tasted.
"V'nitztarfu v'tivlu" - the Terumah and the kil'ei hakerem combined together and seasoned the food.
The ruling of the Mishnah:
"Asur l'zarim u'mutar l'kohanim" - a zar, a non-kohen, is forbidden in both Terumah and kil'ei hakerem alike, and therefore the two prohibitions combine to forbid the mixture to him. But for a kohen it is permitted, since Terumah is permitted to him, and in the spice of kil'ei hakerem alone there is not enough to season the food.
"Rabbi Shimon matir l'zarim v'l'kohanim" - Rabbi Shimon permits the mixture even to a zar, and it is not forbidden to him, even though both Terumah and kil'ei hakerem are forbidden to him. His view is that two different types of prohibitions do not combine with one another.
The question of the commentators: what does this mishnah teach us that is new?
The commentators find it difficult to understand why this mishnah is needed, since it seems to be no more than a repetition of the previous mishnah.
Some explain that spices differ somewhat from leaven: with leaven, although we are dealing with two prohibitions that stem from different passages in the Torah, kil'ei hakerem and Terumah, the type of food itself is one and the same, leaven and leaven. Spices, by contrast, are different kinds of spices from one another, and not all spices are alike. One might therefore have thought that in this case even the first Tanna would agree with Rabbi Shimon and say that the items do not combine. So the mishnah comes and teaches that there is no difference between them: spices and leaven are subject to the very same dispute between the Sages and Rabbi Shimon.
In summary: this mishnah repeats the law of combining taught in the previous mishnah, except that it deals with spice of Terumah and spice of kil'ei hakerem, where neither one alone is enough to season, and together they seasoned the pot. According to the Sages the two prohibitions combine and forbid the mixture to a zar, while to a kohen it is permitted. According to Rabbi Shimon different prohibitions do not combine, and therefore the mixture is permitted to non-kohanim and kohanim alike. The novelty of the mishnah is that even with spices, which are different kinds from one another, the Tannaim disagree just as they disagreed regarding leaven.