Matzah that is made from ma'aser sheni produce, an etrog that is ma'aser sheni produce... doesn't work (following the opinion of R. Meir). Albeit for different rationales for each. Plus, the logic of the formulation of "since" (ho'il) in the Gemara. Also: The Nazir and the matzah of his offering. And the dynamic between R. Ilai and R. Eliezer, as evident in the discussion on these "rekikei nazir" and their knowledge of Halakhah, when some of these details are not explicit in the Torah.