Groundbreaking disagreement in the Gemara?
Last Shabbat, one of the community, who came from Ethiopia, gave a lesson about the different customs an traditions that they had back there at Ethiopia.
He was pointing out several major differences between what the Ethiopian tradition and the Gemara-Rishonim-Shulchan Aruch way of Psika that we practice.(e.g. Sfirat Ha-Omer starts after the seventh day of Passover, rather then the first)
At this point, one of the listeners said that now, that he knows better, he can fix all of the mistakes that they made back there, since they didn't know the Tora ShBeal Pe (Oral tradition).
After the lesson, I told that guy that it is very ignorant of him, and since they have father-to-son tradition all the way back to Bait Rishon (first temple), well before the Mishna, it is legitimate and they should keep it.
I was able to make him change his mind about most of the subjects, except one - the Sfirat Haomer start time. he said that since it is impossible that there was a disagreement about it ever, because Shavuot was celebrated once and not twice, so they must be wrong about it.
I told him that even so, since they have a tradition of a different opinion, it is still valid. [although, when the Jews of Ethiopia came to Israel, they started to count from after the first festival]
I am looking for a similar level of disagreement at the Mishna\Gemara, that is, a disagreement that holds even that there is a established practice that supports one opinion.
The closest I could find is the disagreement about if a Yovel is part of the Shmita cycle, but it is not good enough, since that the cycle count probably ceased at some point.