Towards the beginning of Parashat Tzav, the Torah discusses the basic laws concerning the mincha offering, which consisted of flour mixed with oil. The Torah forbids leavening the mincha – “lo tei’afeh chametz” (6:10) – and this prohibition applies both to the handful that was placed on the altar, and to the portion eaten by the kohanim (Rashi, citing Torat Kohanim). The exception to this rule is the korban shetei ha-lachem, the special sacrifice offered on Shavuot, which included two leavened loaves of bread, as the Torah requires later in Sefer Vayikra, in Parashat Emor (23:17).
The Munkatcher Rav, in Minchat Elazar (5:31), asserts that when a kohen receives both a portion of the shetei ha-lechem and a portion of an ordinary mincha, he must eat them separately. If he eats the two portions together, a piece of the ordinary mincha may absorb chametz in his mouth from the piece of the shetei ha-lechem, and he would thus be in violation of the law forbidding turning the mincha into chametz.
On this basis, the Munkatcher Rav explains the Mishna’s comment in Masekhet Sukka (55b) concerning the case when Shavuot falls on Shabbat. Every Shabbat, the lechem ha-panim – the “showbread” on the table in the Beit Ha-mikdash – was distributed among the kohanim on duty in the Mikdash. And thus when Shavuot fell on Shabbat, the kohanim that day received both a portion of the lechem ha-panim, which was unleavened, and a portion of the leavened shetei ha-lechem. The Mishna states that when these portions were distributed, the kohen was told, “Heilakh matza, heilakh chametz” – “Here is matza, here is chametz.” Meaning, the kohanim was notified that one piece was unleavened, originating from the lechem ha-panim, and the other was chametz, as it was a portion of the shetei ha-lechem. Rashi comments that this notification was given due to the halakha that every sacrifice is distributed equally among the kohanim, and kohanim are not given a larger share of one and then none in another. The kohen was thus informed that he was receiving a portion from each sacrifice as he rightfully deserves. The Munkatcher Rav questions Rashi’s explanation, and argues that a more compelling reason for this procedure is that it serves to warn the kohanim to eat these portions separately. In light of the Torah prohibition against leavening the mincha, it was forbidden for kohanim to eat their portion of a mincha – including the lechem ha-panim – together with chametz, and thus they were notified when they received both chametz and unleavened portions of their respective statuses.
The Munkatcher Rav proceeds to note that the Rambam makes no mention of the Mishna’s comment that kohanim are told, “Heilakh matza, heilakh chametz.” (This omission was noted by the Mishneh Le-melekh in Hilkhot Kelei Ha-mikdash 4:5.) The reason, he suggests, is that this procedure was simply a function of the prohibition against leavening the mincha, which the Rambam codifies explicitly in Hilkhot Ma’aseh Ha-korbanot (12:14). As such, the Rambam did not deem it necessary to mention that the kohanim should be warned to eat the two portions separately.