Menahot 40: Fraudulent Blue

A dispute between Beit Hillel and Beit Shamai on whether a linen cloak is obligated in tzitzit (including tekhelet, that is, which has to be a wool thread). Is that a problem of sha'atnez? The halakhah follows Beit Hillel. So what is the rabbinic decree and how should they publicize it? From what garments need the tzitzit, the Gemara moves to the concern of indigo, which is a plant-based dye and fraudulent when it comes to tekhelet, and that itself received a rabbinic decree -- against people using it and err with regard to the mitzvah inadvertently. Also, the concern of sha'atnez being in a night garment, which itself would be exempt from tzitzit. Plus, the early pious folks who would tie tzitzit on the corners before 4 were even present - which raises the question of "adding" to a mitzvah, which itself is prohibited.