Sanhedrin 27B

קתא דבורטיא.

the handle of a spear [devortiya].

אמר ליה: מאי דעתיך? כרבי מאיר? רבי מאיר ורבי יוסי ⁠– הלכה כרבי יוסי, ורבי יוסי האמר: הוזם בדיני ממונות ⁠– כשר לדיני נפשות!

Rav Abba bar Ya’akov said to bar Ḥama: Why did you bring these witnesses? What do you hold? Do you hold like R' Meir, that one who is guilty of a monetary transgression is disqualified from bearing witness in capital cases too? But in a dispute between R' Meir and R' Yosei, the halakha is in accordance with the opinion of R' Yosei, and doesn’t R' Yosei say that if one was rendered a conspiring witness in a case of monetary law he is fit to testify in cases of capital law?

אמר ליה רב פפי: הני מילי היכא דלא סתם לן תנא כרבי מאיר. הכא סתם לן תנא כרבי מאיר.

Rav Pappi immediately said to him: This statement applies only where the tanna did not teach us an unattributed Mishnah in accordance with the opinion of R' Meir. Here, the tanna taught us an unattributed Mishnah in accordance with the opinion of R' Meir.

ממאי? אילימא מהא דתנן: כל הראוי לדון דיני נפשות ראוי לדון דיני ממונות. מני? אילימא רבי יוסי היא, הא איכא עד זומם דממון, דכשר לדיני נפשות ופסול לדיני ממונות! אלא לאו, רבי מאיר היא.

The Talmud asks: From what Mishnah is it seen that the tanna taught an unattributed Mishnah in accordance with the opinion of R' Meir? If we say it is from that which we learned in a Mishnah (Nidda 49b): Anyone who is fit to adjudicate cases of capital law is fit to adjudicate cases of monetary law, that is inconclusive. Even though one can compare the halakhot of disqualifying judges to the halakhot of disqualifying witnesses, one cannot cite this as an example of a Mishnah taught in accordance with the opinion of R' Meir. Whose opinion is recorded in this Mishnah? If we say it is the opinion of R' Yosei, isn’t there the counterexample of a conspiring witness in monetary matters, who is fit to adjudicate cases of capital law but is disqualified from adjudicating cases of monetary law? Rather, is the ruling of the Mishnah not the opinion of R' Meir?