Orlah, Chapter 2, Mishnah 13. This Mishnah deals with vessels that were smeared with oil, and with the question of how their status is determined when they were smeared both with tamei oil and with tahor oil.
The text of the Mishnah:
"Keilim shesachan b'shemen tamei" - vessels into which one smeared tamei oil. What case is being discussed?
Two ways of understanding which vessels are meant:
Leather vessels: Some understand that the Mishnah is speaking of leather items, such as shoes and the like, which are commonly rubbed with oil in order to soften the leather (as is well known, special oils are sold for softening leather goods).
Other vessels: Others understand that the Mishnah is speaking specifically of other vessels, ones that do not become tamei, thereby excluding leather, since leather itself does become tamei.
In any case, tamei oil was smeared into the vessel, until it was absorbed and dried and was no longer moist, and afterwards - "v'chazar u'sachan b'shemen tahor" - he went back and smeared tahor oil into it. Or the reverse: "o shesachan b'shemen tahor v'chazar u'sachan b'shemen tamei" - that he first smeared on tahor oil and afterwards tamei oil.
The question under discussion:
Do we suspect that the tamei oil will come out of the vessel and render tamei the oil that was smeared on afterwards? For example, in a vessel that is susceptible to tumah, which was immersed in a mikvah after the tamei oil was smeared on it, and afterwards tahor oil was smeared on it, and likewise in the reverse case, where tahor oil was smeared on first and tamei oil afterwards.
The dispute between Rabbi Eliezer and the Chachamim:
Rabbi Eliezer says: "Achar harishon ani ba" - we must be concerned that even the first oil may come out. If the tamei oil was the second one, it is obvious that we must be concerned and consider it tamei; but even if the tamei oil was the first one, and the vessel was then immersed in a mikvah and afterwards tahor oil was smeared on it, we do not assume that the tamei oil did not come out at all. Rather, we assume that it did come out, and therefore it causes tumah.
And the Chachamim say: "Achar ha'acharon" - we follow only the oil that entered last. Whatever entered first and then dried out, we do not suspect that it will come out again. Therefore, when tamei oil was smeared on first and tahor oil last, it is tahor.
In summary: The Mishnah deals with vessels that were smeared with tamei oil and with tahor oil one after the other, after the first oil had been absorbed and had dried. Rabbi Eliezer and the Chachamim disagree: according to Rabbi Eliezer, "achar harishon ani ba" - we are concerned even for the first oil, that it will come out and cause tumah; and according to the Chachamim, "achar ha'acharon" - we follow the oil that was smeared on last, and we are not concerned about oil that has already dried.