The Nazir is the exception to the rule TB Nazir 42

Today’s daf Nazir 42 discusses two important halakhic principles and how they relate to the nazir. The nazir is an exception to the general rule “”The majority of an entity is considered like all of it- רוּבּוֹ כְּכוּלּו). You might think with this principle that when a nazir shaves his entire head, but leaves two hairs, he has fulfilled his the mitzvah of shaving his head because he shaved 99.99% of all of his hair. You would be wrong. “This principle is derived from the fact that the Merciful One revealed in the Torah and specified with regard to a nazirite: “On the seventh day he shall shave it” (Numbers 6:9), despite the fact that the same verse already stated: “And he shall shave his head on the day of his cleansing.” This teaches that it is only in this case here that he does not fulfill the mitzva of shaving until there is the removal of all of it, i.e., shaving part of his head is insufficient.” (Sefaria.org translation)

When we studied massekhet Shabbat we learned the principle that one is not held liable when the outcome of an unintended action violates a prohibition. We know that until the nazir completes the days of his vow, he is forbidden to even remove one hair on his head. Nevertheless, the Mishnah says “A nazirite may shampoo [ḥofef ] his head and separate [mefaspes] his hairs manually, without concern that hairs might fall out…” (Sefaria.or translation) The Gemara gives the reason why. “Rabbi Shimon, who says: An unintentional act is permitted (דָּבָר שֶׁאֵין מִתְכַּוֵּין מוּתָּר). Even if hairs do fall out as a result of this action, as he did not intend this to happen the action is permitted.”(Sefaria.org translation) According to Rabba, Rabbi Shimon would forbid a nazir from combing his hair because his intention is to remove the loose hairs on his head. “…anyone who combs his hair intends to remove stray hairs, and therefore this is considered an intentional act.” (Sefaria.org translation)