Some posters on firstname.lastname@example.org have been claiming that the Ramban and Rabbeynu Yona (acc. to some, as cited in Shitta MiKUbetses) indicate in their comments on Bava Basra 131a that there are instances where Amoraim disagree with Tannaim. Do you see any indication of this, beyond the fact that the amoraim routinely posken like one Amora over another?
Zvi Lampel, Passaic, NJ; USA
SHITA MEKUBETZES 131a DH OMAR LEI RAV PAPA (second one) on p.281 col. 2 does say explicitly that even though it is not the way of the Amora'im to disagree with the Tana'im, this is only on something stated explicitly in a Mishnah or a Beraisa, which implies that they can differ with Tana'im on something not stated explicitly in a Mishnah or Beraisa.
Even though the Gemara sometimes says "RAV TANA U'POLIG" - Rav is considered like a Tana and may disagree with the Tana'im - which implies that anyone with less authority than Rav cannot disagree with the Tana'im, however, I think the Gemara usually illustrates this special power of Rav in connection with disagreeing with an explicit Mishnah.
See also BI'UR HA'GRA SHULCHAN ARUCH EH 80:18 who cites a case where RAMBAM maintains that the Gemara disagrees with the Mishnah. See also ROSH BAVA BASRA 1:42 that an Amora can rule according to a minority opinion in Tana'im if he is aware of the dispute. Shita Mekubetzes KESUBOS 81a, p.183 DH VEHA'TANYA says the same thing in the name of RASHBA. However, even though there might be some special cases where Amora'im can disagree with Tana'im, I think this is a rare occurence.