After the second Temple's destruction, there is a discussion of Passover that falls on Shabbat and there is an astounding debate on the Passover offering and Shabbat.
How could it be that the Sages did not know whether the Passover offering can override Shabbat?
How can it be that the Sons of Bteira, who seem to have been the authorities at the time, "forgot the law"?
The people told the Sons of Bteira of an immigrant from Babylon who might know the answer. The rest is Jewish Talmudic history.