A Jewish Nazarite, according to the Torah, is forbidden to drink wine or to get a haircut, among other prohibitions and laws he must obey. After his Nazarite period ends — generally at the end of thirty days — he must shave his entire head. He is forbidden to leave even two hairs on his head, for that would not be considered shaving the entire head.
One of the sages of the Talmud asks: What is the rule for a Nazarite who shaved all his head but two hairs and then hairs grew back; in this new state he shaves off the two hairs which he had not previous shaved? Is this considered shaving his entire head? In the final analysis he did shave the two hairs which were left!! Or, because new hair had already sprouted, does it not help that he shaved the last two hairs?
The sage also added another question, wondering what the rule is for a Nazarite who shaved his head and left those two hairs. Then one hair falls out on its own, and he shaves off the last. Would it be held that the Nazarite fulfilled the commandment of shaving? In the final analysis no hairs are left on his head! But since he did not actively shave his entire head, some fell out on its own, would it not be considered shaving his head?
(Babylonian Talmud, Tractate Nazir 42a)