“Joseph: squeak: Going with your pshat, how is it that it is still mutur to kill (what we call) lice on Shabbos?”
The answer to this is pretty straightforward–it’s a pashut Gemara in Masechet Rosh Hashana 25a. Rabban Gamliel poskined when Yom Kippur would start. Rabbi Yehoshua pointed out that RABBAN GAMLIEL WAS MISTAKEN IN HIS ASTRONOMICAL CALCULATIONS. Rabban Gamliel responded by decreeing that Rabbi Yehoshua must come before him with his money bag on the day that Rabbi Yehoshua had claimed should be Yom Kippur; Rabban Gamliel was the leader, and would be the one to set when Yom Kippur is, and Rabbi Yehoshua was supposed to defer to this.
Rabbi Akiva comforts Rabbi Yehoshua by reading the word “otam” in “tikreu otam b’moadam” as “atem,” interpreting the three uses of this word in the Torah as: “atem”–you set when the holidays are. “Atem afilu shoggegin, atem afilu mezidin, atem afilu muta’in.” “You [set it] even if mistaken, you even if purposefully wrong, you even if you make an error.”
The message is simple. Chazal’s role is to determine halacha and Hashem’s will for humans. To this extent, their word becomes law in the halachik realm, even if they base it on a scientific error. This is, after all, the clear point of the Gemara; Rabban Gamliel was mistaken scientifically, but his psak was followed anyway, because he is a spiritual leader, not a scientific one. If you think that saying the sages could be mistaken in science takes away from their infallibility in halacha, you have simply misunderstood their role in both.