“U’shmartem es hamatzos” (Shemos 12:17) teaches that there is a requirement for matzah to be guarded/watched (Pesachim 38b). Rashi (ibid.) explains that this watching (shmirah) has two parts. First, one must watch the matzah to ensure that it does not become chometz. Second, the shmirah itself needs to be for the sake of matzah shel mitzvah. I would like to elaborate on these two elements.
Regarding the first element, although there is a general principle to guard oneself from sinning, such as guarding one’s tongue from speaking lashon harah, there is a specific halacha that matzah must be watched so that it does not become chometz. Based on this halacha, Rabbi Eliezer forbids eating matzah made by a Kusi (those who converted in a questionable manner in the times of the King of Ashur.) Kusim are not aware of the halachah that matzah must be guarded from becoming chometz. Thus, their matzah are not suitable for Pesach (Chulin 4a).
The Gemara in Pesachim (102a-b), amidst a lengthy discussion of the various laws of Kiddush and Berachos, quotes the following Beraissa: Our Rabbis taught, members of a group who were reclining and Shabbos began while they were still involved in their meal — Rebi Yehudah says, we bring a cup of wine and say Kiddush over it. Rebi Yosi says, the group may continue eating after dark. When they finish, they should say Birchas HaMazon on one cup of wine, and then Kedushas HaYom on another. The Gemara asks, why is it necessary to make Birchas HaMazon and Kiddush on two separate cups of wine? Let the mevareich say both on one! Rav Sheishes cryptically responds, because we do not say two kedushos on one cup. The Gemara asks, what is the reason, to which Rav Nachman responds: ein osin mitzvos chavilos chavilos — we do not make miztvos into bundles.
The Gemara in Sotah (8a) quotes a beraissa as the source for this principle: We don’t make two Sotos drink the mei sotah at once; we don’t purify two metzora’im at once; we don’t pierce the ears of two avadim at once; nor do do we break the neck of two calves at once, since we don’t make mitzvos chavilos chavilos.