Recently, on Bava Kamma 25a, we encountered the following:
וּמַאן תַּנָּא דְּשָׁמְעַתְּ לֵיהּ דְּאָמַר: שִׁכְבַת זֶרַע שֶׁל זָב מְטַמְּאָה בְּמַשָּׂא? לָא רַבִּי אֱלִיעֶזֶר, וְלָא רַבִּי יְהוֹשֻׁעַ. דִּתְנַן: שִׁכְבַת זֶרַע שֶׁל זָב מְטַמְּאָה בְּמַגָּע, וְאֵין מְטַמְּאָה בְּמַשָּׂא; דִּבְרֵי רַבִּי אֱלִיעֶזֶר. וְרַבִּי יְהוֹשֻׁעַ אוֹמֵר: אַף מְטַמְּאָה בְּמַשָּׂא, לְפִי שֶׁאִי אֶפְשָׁר בְּלֹא צִחְצוּחֵי זִיבָה.
Once the Gemara raised the issue, it clarifies: And who is the tanna about whom you heard that he said: Semen of a zav imparts ritual impurity by carrying? It was not Rabbi Eliezer and not Rabbi Yehoshua. As we learned in a baraita: Semen of a zav imparts ritual impurity by contact but it does not impart ritual impurity by carrying; this is the statement of Rabbi Eliezer. And Rabbi Yehoshua says: It also imparts ritual impurity by carrying, as it is impossible for semen to emerge without small drops of gonorrhea-like discharge [ziva] accompanying it.
Note the bolded with ditnan, even though what is being cited is a brayta. This is strange because the usually terminology to cite a brayta is ditanya, and to cite a Mishnah is ditnan.
As we keep reading, the next statement in the gemara is this:
עַד כָּאן לָא קָאָמַר רַבִּי יְהוֹשֻׁעַ הָתָם – אֶלָּא שֶׁאִי אֶפְשָׁר בְּלֹא צִחְצוּחֵי זִיבָה, הָא לָאו הָכִי – לָא! אֶלָּא הַאי תַּנָּא הוּא – דִּתְנַן: לְמַעְלָה מֵהֶן
Rabbi Yehoshua says there that the semen of a zav imparts ritual impurity by carrying only because it is impossible for semen to emerge without small drops of ziva accompanying it. This indicates that if not for this reason, the semen would not impart ritual impurity by carrying, according to the opinions of both Rabbi Yehoshua and Rabbi Eliezer. The Gemara explains: Rather, it must be that this tanna is the one who holds the opinion that the semen of a zav imparts ritual impurity by carrying, as we learned in a mishna listing the sources of ritual impurity (Kelim 1:3): Of a greater degree than the ritual impurities listed previously in the mishna, i.e., the impurity of a creeping animal, semen, and one who contracted ritual impurity from a corpse,
Here, ditnan is correct, for it refers to a Mishnah. The logical explanation is that some scribe along the way got confused, and the later ditnan influence this citation to also become ditnan.
Artscroll has a writeup of the issue, in footnote 29:
There are two editors working here. The unbracketed text is first, and he notes the issue, that it is a brayta being cited, not a Mishnah. The second editor, in [square brackets], proposes a solution. Pointing us to Tosafot on Chullin 87b, d”h ditnan, and Rashba to Chullin 14a d”h ditnan halokeach, there is a certain fluidity to these introductions.
While this is well and good in general, it pays to check the particulars of each case. Here, it is a clear error in printed editions. Here is Vilna, Soncino, and Venice. Vilna and Venice have the incorrect ditnan, and the earlier Soncino have ditany’. The final aleph was omitted, instead having an apostrophe. We can easily imagine that turning into a nun sofit.
Meanwhile, all of the manuscripts have ditanya:
So, this isn’t something we need to justify by awkward fluidity of terms. Some copyist for Vilna erred, or else just copied from the error in the Venice printing.