Yevamot 43A

וכי רבי לא שנאה, רבי חייא מנין לו?

If R' Yehuda HaNasi had not taught that ruling in the Mishnah, from where would R' Ḥiyya, his disciple and redactor of the baraitot, have known it? Since the source of the unattributed ruling in the baraita is certainly the dispute recorded in the Mishnah, its lack of attribution in the baraita only reflects the fact that R' Ḥiyya ruled in accordance with that opinion, but that cannot be used to indicate that it is the accepted halakha.

אמר ליה, והא תנן: מסרק של פשתן שניטלו שיניו, ונשתיירו בו שתים ⁠— טמאות, ואחת ⁠— טהורה. וכולן שניטלו אחת אחת בפני עצמן ⁠— טמאות.

R' Naḥum questioned the principle that the halakha is always in accordance with an unattributed ruling in a Mishnah and said to R' Ḥiyya: Didn’t we learn in a Mishnah (Kelim 13:8): A comb made for combing beaten flax in preparation for spinning, from which its teeth were removed and only two of its teeth remained in it, is still suitable for combing. It is therefore considered a utensil and will become ritually impure if it comes in contact with ritual impurity. However, if only one tooth remained, so that the comb was no longer suitable for combing, it is no longer considered a utensil and so will remain ritually pure even if it comes in contact with ritual impurity. And with regard to any of the teeth that were individually removed on their own, since they do serve a purpose, e.g., they may be embedded and used as hooks, they are considered utensils and can become ritually impure.

של צמר שניטלו שיניו אחת מבינתים ⁠— טהור. נשתיירו בו שלש במקום אחד ⁠— טמא. היתה החיצונה אחת מהן ⁠— טהור. ניטלו שנים ועשאן למלקט ⁠— טמאות. אחת והתקינה לנר או למיתוח ⁠— טמאה.

A comb made for combing wool from which every other one of its teeth were removed, so that no two consecutive teeth remained in place, is no longer suitable for combing and so will remain ritually pure. If 3 teeth remained in it in a single place so that it could still be used for combing, then it could become ritually impure. However, if one of those teeth was the external frame of the comb itself, then it could not function as a comb, and so it will remain ritually pure. If two of the teeth were removed and made into small pincers, then they could become ritually impure. Alternatively, if one of the teeth was removed, and it was prepared in a manner that made it suitable for use in cleaning an oil lamp or for stretching ropes, then it would be considered a utensil and could become ritually impure.