Shevi'is, chapter 8, mishnah 7. The mishnah rules that one may not cook a vegetable of shevi'is, which carries the sanctity of shevi'is, in oil of Terumah. What is the reason for this prohibition? So as not to bring the vegetable to a state of disqualification, where it can no longer be used.
The reason for the prohibition according to the Rambam:
According to the Rambam, because of the sanctity of Terumah it must be eaten in purity, and once it becomes tamei it can no longer be eaten. There is therefore a concern that the Terumah oil absorbed in the shevi'is vegetable will become tamei, and then the shevi'is vegetable can no longer be used - a completely unnecessary act of ruin. Better, then, to use ordinary oil rather than oil of Terumah, so as not to reach a situation in which a person will ultimately be forced to destroy a vegetable that carries the sanctity of shevi'is.
The view of the Rash - the opposite concern:
The Rash is cited here, and his understanding differs from the Rambam's: in his view the concern is the reverse - that the shevi'is will render the Terumah oil unfit to eat. For shevi'is is subject to the law of bi'ur, and once the time of bi'ur arrives the produce may no longer be eaten, and from then on that restriction will apply to the Terumah as well.
It should be noted that the Rash's understanding is difficult, since the subject under discussion here is the sanctity of shevi'is: that is the topic of the tractate, and of this section of it in particular, and not the prohibition of ruining Terumah (even though that too is a halachah). Since the focus is on the status of the shevi'is, the Rambam's words seem easier to understand.
"Rabbi Shimon meitir" - "Rabbi Shimon permits":
The mishnah continues: "Rabbi Shimon meitir" - Rabbi Shimon permits cooking the shevi'is vegetable in oil of Terumah, and this is his position throughout the Talmud: one may bring sanctified items into a situation of potential disqualification. It is permitted to place holy things - whether offerings, Terumah or shevi'is - into a situation where they may ultimately become disqualified. Therefore one may cook: true, the item may become disqualified, and if it does, it does, but one is permitted to take that risk.
"Acharon acharon nitpas bashevi'is, u'fri atzmo asur" - "each last item is caught up in shevi'is, and the fruit itself remains forbidden":
The mishnah concludes with an interesting halachah concerning the transfer of the sanctity of shevi'is: the last item exchanged for the shevi'is produce is caught up in shevi'is and takes on the halachic status of the sanctity of shevi'is, while the fruit itself - the original fruit with which we began - remains forbidden.
This can be illustrated with a chain of exchanges:
He exchanged shevi'is produce for meat: both the meat and the produce stand in the sanctity of shevi'is - the produce never loses its status, as the mishnah says, "u'fri atzmo asur" - "and the fruit itself remains forbidden", and the meat exchanged for it takes on the sanctity of shevi'is. The law of bi'ur applies to both, and both must be treated with the sanctity of shevi'is.
He exchanged the meat for fish: the meat loses its status, and the fish are now the ones caught up in the sanctity of shevi'is, while the original fruit remains in its sanctity, as the mishnah states.
He exchanged the fish for oil: the fish are no longer in the sanctity of shevi'is, and the oil is caught up in it. From now on two things must be disposed of: the original produce and the oil.
This is the meaning of the rule "acharon acharon nitpas bashevi'is" - the last item in the chain is the one caught up in the sanctity of shevi'is; and even so, "u'fri atzmo asur" - the original fruit remains forbidden in its sanctity.
In summary: in this mishnah we learned the prohibition against cooking a vegetable of shevi'is in oil of Terumah - according to the Rambam out of concern that the tumah of the oil will bring the shevi'is vegetable to disqualification, and according to the Rash out of concern that the law of bi'ur of shevi'is will restrict the Terumah; the view of Rabbi Shimon, who permits it because one may bring sanctified items into a situation of potential disqualification; and the law of the transfer of sanctity through exchanges - "acharon acharon nitpas bashevi'is, u'fri atzmo asur".