Keritot, Chapter Five, Mishnah Six
In todays mishnah, he is not sure whether he ate forbidden fat, for which he would need to bring a hatat, or consecrated, permitted fat, for which he would need to bring an asham.
For the parts of this mishnah that have already been explained above, I will just make reference to those mishnayot.
1) If there was a piece of forbidden fat and a piece of consecrated [permitted fat], and a person ate one of them and does not know which, he is liable to an asham talui;
2) If he then ate the second piece, he is liable to a hatat and a certain asham.
3) If he ate the one piece and another came and ate the other, each of them brings an asham talui.
a) Rabbi Shimon holds: they together bring a hatat and an asham.
b) Rabbi Yose: two people cannot together bring one hatat and one asham.
Section one: He brings the asham talui for the fact that he might have eaten forbidden fat. For Rabbi Akiva who holds that one brings an asham talui for a case of doubtful sacrilege, the asham talui also covers the possibility that he ate consecrated fat. In other words, one asham talui can cover two possible sins, since we know he only did one of them.
Section two: In this case we know that he ate forbidden fat and consecrated fat, so he must bring a hatat and an asham.
Section three: See mishnayot 4-5.