Do the Rabbinic Sages have the power to uproot a Torah law?
The Gemara on our daf suggests that we find a case of this in the first Mishna of our perek. The Mishna teaches that in a case where a man travels overseas and is reported dead, and the widow remarries based on the permission that she gets from the beit din, should the husband reappear, she is forbidden to them both, and both husbands must write her a get (a divorce). Furthermore, children that she has with either of these men will be considered mamzerim – children born from an adulterous relationship.
There are differences, however. It is clear that any child born from the second husband is a mamzer, since he was living with a married woman – albeit based on misinformation. Future children who are born from the first husband, however, should not be considered mamzerim – after all, the woman did not engage in forbidden relations on purpose. Thus, the ruling that these children are mamzerim is only rabbinic.
The Gemara on our daf argues that declaring someone to be a mamzer when this is not true on a Torah level is effectively giving the Sages power to uproot a Biblical law. This is because ruling that someone is a mamzer will prohibit them from marrying a member of the larger community, but will permit them to marry a mamzeret – a female mamzer – something that is really forbidden.
The Gemara responds that this does not prove the point since both Shmuel and Rabbi Yohanan understand the ruling in the Mishna declaring him a mamzer to apply in a stringent fashion in all directions – i.e. to forbid him from marrying not only a woman from the larger community, but also a mamzeret, as well.
Although this answers the immediate question from our Mishna, the larger question of whether the Rabbinic Sages have the power to uproot a Torah law is the topic of an extended discussion in our Gemara.