Nazir, Chapter Three, Mishnah 4. The Mishnah deals with someone who accepted upon himself a long nezirus - "Hareini nazir me'ah yom" - "I am a nazir for a hundred days" - and became tamei at the end of his days of nezirus. There are two cases before us: one who became tamei on the hundredth day, and one who became tamei on the hundred and first day.
He became tamei on the hundredth day:
According to the first Tanna, he cancels the entire nezirus. The reason: since he must complete a full hundred days, he is not permitted to shave and bring his offerings until the hundred and first day, and if he did so on the hundredth day, it does not count after the fact. It turns out that tumah on the hundredth day has the status of tumah in the middle of the nezirus, and therefore it cancels everything, and he must begin again.
Rabbi Eliezer disagrees and says: "Eino soser ela sheloshim" - he cancels only thirty days.
Why? Seemingly, as we saw in the previous Mishnah, one who vowed a nezirus of thirty days and became tamei on the thirtieth day cancels his entire nezirus. However, Rabbi Eliezer holds, based on a derivation from the verse, that in a long nezirus of more than thirty days, one who became tamei on the very last day of his nezirus cancels only thirty days. The derivation does not uproot the rule that tumah in the middle of the nezirus cancels everything; rather, it teaches that when the tumah occurred on the very last day, the Torah imposed only thirty days upon him.
In the previous Mishnah, where the entire nezirus was thirty days, this concept had no practical expression, since the number of days cancelled is thirty in any case. But in a long nezirus of a hundred days, the derivation makes a difference: one who became tamei on the last day of his nezirus cancels only thirty days.
He became tamei on the hundred and first day:
According to the first Tanna, "soser sheloshim yom" - he cancels thirty days, and only thirty days.
And why should he cancel anything at all? After all, on the hundred and first day the nezirus has already ended, and the tumah occurred afterward. Rather, this is a decree of the Sages: since tumah on the hundredth day cancels the entire nezirus, they decreed regarding the hundred and first day as a safeguard for the hundredth day. But they were not stringent to cancel a hundred days, only thirty days.
Rabbi Eliezer disagrees and says: "Eino soser ela shivah" - he cancels only seven. In practice there is no cancellation here at all, but rather a wait of seven days until he becomes tahor, and once he is tahor he brings the offerings of his nezirus.
In summary: In this Mishnah we learned the law of one who vowed a nezirus of a hundred days and became tamei at the end of his nezirus. If he became tamei on the hundredth day, according to the first Tanna he cancels everything, since the completion is not finished until the hundred and first day, while according to Rabbi Eliezer he cancels only thirty days, based on the derivation from the verse regarding a long nezirus in which he became tamei on the last day. If he became tamei on the hundred and first day, according to the first Tanna he cancels thirty days, by a decree of the Sages as a safeguard for the hundredth day, while according to Rabbi Eliezer he cancels only seven, meaning that he waits until he becomes tahor and brings his offerings.