Menachot 40 - Beit Shammai Do Not Allow Shaatnez Anywhere

We learned earlier that the forbidden mixture of wool and linen, or shaatnez, is allowed in tzitzit, because the two commandments follow each other in the Torah, forming a “this is prohibited, [but] this is allowed” pattern. However, this is only the opinion of Beit Hillel. Beit Shammai, who do not derive conclusions from the proximity of phrases, consequently do not allow Shaatnez in the tzitzit.

But practically, anybody who would put a blue woolen thread in his linen tzitzit, aroused astonishment in Jerusalem of old. Jerusalem was the place of especially pious people, and they did not use the Beit Hillel's permission. Why not? Must be that in practice Beit Hillel also forbade this.

Why would Beit Hillel forbid what they said was allowed by the Torah? Could it be because of the ignorant people who would not know the difference? Then let ten individuals wear it in the manner it is allowed! - All the more they will be astonished. Then let's teach them! - That would work, so the reason must be different. Talmud continues to look for the reason, and you are invited to submit your guesses.

Art: Jose Frappa - Surprised