Nazir, Chapter 5, Mishnah 6. This mishnah is a direct continuation of the previous one, in which several men accepted nezirus upon themselves depending on the identity of a man who was approaching them: one said that he was accepting a vow of nezirus if this was so-and-so, and his friend said that he would be a nazir only if it was not so-and-so.
"Hirtia la'achorav":
The mishnah deals with a case in which the man who was approaching them turned back, and his identity was never established. As a result, both of those who vowed remain in doubt, and there is no way to clarify whether the vow took effect.
The opinion of the Tanna Kamma:
The Tanna Kamma rules "eino nazir" - none of them is a nazir, since each one of them stands in a state of doubtful nezirus, and in such a case we apply the principle that a doubtful nezirus is treated stringently. This is the opinion of the Tanna Kamma.
The opinion of Rabbi Shimon:
Rabbi Shimon disagrees and holds that since the person is considered a doubtful nazir, he should conduct himself the way one conducts himself in any case of doubt, and say:
"Im hayah kidvarai - hareini nazir chovah" - if I was indeed correct, the vow of nezirus that I made is valid, and I am therefore a nazir by virtue of the obligation of my vow.
"Ve'im lav - hareini nazir nedavah" - and if I was not correct, I hereby accept nezirus upon myself voluntarily.
In this way he is covered in either case: whether the nezirus comes by force of his original vow or by force of the acceptance he is making now, he will be able to conduct himself as a nazir.
In summary: in a case where it was never clarified who the man approaching them was, the Tannaim disagree: according to the Tanna Kamma, neither of those who vowed is a nazir, while according to Rabbi Shimon a person removes himself from the doubt of nezirus by stipulating an obligatory nezirus for one possibility and a voluntary nezirus for the other.