Nedarim 27a: Who is the Tanna of this baraisa that the Rabbis taught? “If one vowed not to benefit from five people, and he annulled the vow for one of them, all of them become annulled. But if he originally made the vow excluding one of them, he is permitted and the rest are forbidden.”
נדרים כז ע”א: מאן תנא להא דתנו רבנן: נדר מחמשה בני אדם כאחד ־ הותר לאחד מהם הותרו כולן, חוץ מאחד מהן ־ הוא מותר והן אסורין.
Two boys in the Mirrer Yeshiva were sitting in the Beis Medrash, Gemaras open on their shtenders, but they were schmoozing and wasting time. Suddenly, Reb Nochum walked by. The boys quickly pretended to be learning, chanting loudly, “Tanu Rabanan…” Reb Nochum said, “I know you’re not learning, because there is no Tanu Rabanan anywhere in Nedarim!”
When this story was told to Rav Chaim Kanievsky zt”l, he commented, “It’s true that there is no Tanu Rabanan, but there is one D’tanu Rabanan.”
Source: FJJ March 25, 2022 p. 44
[There are only two masechtas where “Tanu Rabanan” does not appear: Nedarim and Tamid. Otherwise, it appears 1,979 times, which is on average once every 1.3 blatt. There is clearly a significant difference here. As Tosafos says (Nedarim 7a), Nedarim is written in a different style (לשון נדרים משונה). The Tiferes Yisroel on Avos 2:4, Boaz, writes about the virtues of chazarah and explains the Mishnah אל תאמר לכשאפנה אשנה as referring to chazarah. And an author must certainly check over his sefer to eliminate errors. Even the great Rav Ashi with his yeshiva went through Shas twice in his life, each time for 30 years, in order to compile and perfect it. The masechtas Nedarim, Nazir, Erechin, Krisos, Temurah, Me’ilah and Tamid, he proposes, have a different style because Rav Ashi didn’t get to edit them a second time. We can theorize that maybe in his second edition he decided to distinguish a certain kind of Baraisa from others using the code words “Tanu Rabanan”. Nedarim, since it is from the first edition, does not contain these words.]