Maseches Shevuos, chapter two, mishnah 5. This mishnah deals with a dispute among the Tannaim over the question of which forgetting obligates a person to bring a korban oleh v'yored: forgetting the tumah or forgetting the Mikdash.
The view of Rabbi Eliezer:
Rabbi Eliezer relies on the wording of the verse: "o b'nivlas sheretz tamei v'nelam mimenu" - this speaks of a person who touched one of the things that convey tumah, such as a dead sheretz which is tamei, and the matter slipped his mind. From here Rabbi Eliezer rules: "al helem sheretz chayav, v'eino chayav al helem Mikdash" - he is liable when he forgot the matter of the tumah itself, and he is not liable when he forgot that this place is the Beis HaMikdash. This ruling stands in contrast to what we learned in mishnah 1 of this chapter, according to which even one who forgets the Mikdash is obligated to bring a korban oleh v'yored.
The view of Rabbi Akiva:
Rabbi Akiva disagrees with Rabbi Eliezer on one point only. He relies on the verse "v'nelam mimenu v'hu tamei", and from here: "al helem tumah chayav, v'eino chayav al helem Mikdash". He too repeats that there is no liability for forgetting the Mikdash, but he uses the expression "forgetting the tumah" rather than "forgetting the sheretz".
What is the point of their dispute?
The Gemara explains, according to one of the opinions, what lies at the root of the dispute:
Rabbi Eliezer, who says "forgetting the sheretz", holds that a person must know the particular thing that caused his tumah: was it a sheretz or the carcass of an animal? He must know exactly what brought him to a state of tumah.
Rabbi Akiva holds that he need not know what caused his tumah. As long as he knows that he is tamei, he can be obligated to bring a korban oleh v'yored.
On one point, however, both are in agreement, as both stated explicitly: there is no liability for forgetting the fact that this area is part of the Beis HaMikdash.
The view of Rabbi Yishmael:
Rabbi Yishmael disagrees with them on this point, and derives his view from the fact that the verse says "v'nelam... v'nelam" - twice. The redundancy in the expression "v'nelam" comes to teach us that one is liable even for forgetting the tumah itself, and it appears that he agrees with the principle that one need not know the particular tumah, but it is enough that he is tamei, as Rabbi Akiva holds. Rabbi Yishmael, however, adds: "chayav al helem tumah v'al helem Mikdash" - even if he forgot about the Mikdash and remembered that he was tamei, even then he is obligated to bring a korban oleh v'yored.
In summary: in this mishnah we learned three views. Rabbi Eliezer holds one liable specifically for forgetting the sheretz, that is, when he knows the particular source of the tumah, and exempts him for forgetting the Mikdash. Rabbi Akiva holds him liable for forgetting the tumah alone, even without knowledge of the particular source, and he too exempts him for forgetting the Mikdash. Rabbi Yishmael derives from the doubled expression "v'nelam... v'nelam" that one is liable in both cases alike - both for forgetting the tumah and for forgetting the Mikdash.