[Avodah] The Rosh ruled that a rav was a zaken mamre

[Shmuel Weidberg]

Look in Shut HaRosh 21:8 which I saw quoted in Rabbi Bechofer's sefer
on contemporary eruvin.

Somebody wrote the Rosh that upon the passing of the former Rav of
Fereres that Rav's talmid became Rav and disallowed the eruv that had
been there under the previous Rav. The Rosh wrote back that the new
Rav had misunderstood a mishna and that the new Rav must reinstate the
Eruv. The new Rav disagreed with the Rosh and felt that he had to be
machmir to not replace the Eruv.

The Rosh told him that he had no right to understand the halacha in
the way that he was understanding it because all the poskim understood
it differently and he was making the mistake and all the poskim were
right.

Eventually the Rosh sent him a letter that was to be received by him
in the presence of two witnesses after which he would have two weeks
to replace the eruv and if not, he would be put in cherem. As well, he
told him that if it was in the time of the Sanhedrin he would be
considered a zaken mamre.

At the same time time he wrote a letter to the original complainant
and told him to deliver the letter and that if the rav did not listen
and did not accept the cherem he should present the letter to the
king's representative and tell him to fine him 1000 coins as the Rosh
had the right to punish people under authority of the king. If that
still didn't work, he was to report back to the Rosh and the Rav was
to be treated as a zaken mamre, which seems to imply that there was a
serious option of the Rosh actually sentencing him to death.