Nazir 54 - Not to shave!
There are cases when the ritual impurity does not really exist, but it was "decreed by the Sages." Take, for example, a field where a grave was previously located, then the location of it was lost, and then the field was plowed over. Since anyone who is walking through the field can potentially touch a bone and become impure, the Sages said that such a field should be treated as ritually impure, and anyone who went through it without precautions will have to purify himself.
However, from the Torah point of view, accidentally touching a bone is unlikely, because a plow does not usually reach that deep, and so really the person is pure. A nazir who becomes impure should cut his hair and bring sacrifices, but a nazir who is pure is not allowed to touch even one hair. What is a nazir to do if he passed through a "field full of fragments" as it is called?
Here is what he should do: he should purify himself with the ashes of the red heifer on days number three and seven, and then continue being a nazir. The seven days of his impurity are taken out from his count, but otherwise, he continues to count as before. And he does not shave his hair or bring sacrifices, because in truth he is pure. Other similar cases, such as being under the same branch as a corpse (but not a real roof that transmits impurity) are treated the same way.
If, in addition to being a nazir, he became a metzora (spiritual leper), he purifies himself from that, but continues being a nazir, since metzora, even though it requires purification, is not the same as the impurity of the dead. Thus, he does not loose all the previous days, as he would if we were to go to a cemetery.
Art: Plowing ox By Rudolf Koller