The first Mishna of Horayos clearly holds like R'Yehuda that an individual is patur if he follows beit din and is talui b bet din. How can the Rambam posken against the misnah even if the gmara brings a view of the rabanan in their discussion. Can we not assume Rebbe also knew the position of the chachamim and he choose to ignore it and poskin as stated in the Mishna. Does the Rambam or others often posken against the mishna if a contrary opinion is not mentioned in the mishna? If so by what halachik athourity can he poskin against the Mishna.?
Toda,
Avi Weinstein, Efrat, Israel
The Poskim very often Pasken against the Mishnayos. This is due to the fact that Halachah is not based on the Mishna, but rather on the Gemara. For more information on this concept, see the RASHBAM in Bava Basra (130b, DH "Ad she'Yomru Lo").
All the best,
Yaakov Montrose