Peah, Chapter 6, Mishnah 3. This mishnah deals with additional cases in which everyone agrees, including Beis Hillel, that they are not considered shikhechah (forgotten sheaves) - because there is something about them that causes a person to remember the sheaf, even if he forgot it for a short while.
The three cases in the mishnah:
"Rashei shuros" - the ends of the rows. Our mishnah does not explain what this means; that explanation is left to the next mishnah, which will spell out what "rashei shuros" are.
"Ha'omer shekenegdo mochiach" - a sheaf whose counterpart across from it serves as a marker for it and calls attention to it.
"Ha'omer shehechezik bo leholicho el ha'ir ushechacho" - a sheaf that a person took hold of and acquired in order to carry it to the city, and afterward forgot it.
In each of these cases the mishnah rules: "Modim she'eino shikhechah" - even Beis Hillel agree that these are not included in shikhechah.
Explaining "the sheaf whose counterpart calls attention to it":
Picture a case of one hundred sheaves of grain, arranged ten sheaves from east to west and ten sheaves from north to south, like a large square. A person gathering the sheaves going in the east-west direction moves along, forgets one of them and continues on - seemingly we could say that he forgot it. But since that sheaf also stands in a complete row running from north to south, we say "the sheaf whose counterpart calls attention to it": the second direction shows that he is still going to come back and gather it. For even if he forgot it while walking from east to west, when he walks from north to south he will remember it, since it is part of a complete north-south row. It turns out that he forgot it only with respect to the east-west pass, and not with respect to the north-south pass.
The third case and its dependence on the versions of the previous mishnah:
The last case, "a sheaf that he took hold of in order to carry it to the city," fits well with the view of Beis Hillel if we learn the previous mishnah according to the version of Rabbi Yehoshua. According to that version, Beis Hillel hold that the mere fact that something is lying next to another item is not enough to remove it from the category of shikhechah. Here, however, where the person took hold of it and afterward forgot it, our mishnah teaches that according to Beis Hillel this is not shikhechah, because he took hold of it.
But if we learn the previous mishnah according to the version of Rabbi Eliezer, who holds that Beis Shammai and Beis Hillel disagreed in a case where he took hold of it, and Beis Hillel say that even when he took hold of it, it is considered shikhechah - this does not fit with our mishnah. For our mishnah rules that the very act of taking hold of the sheaf, before he forgot it, is enough to say that this is not shikhechah and it does not belong to the poor.
In summary: This mishnah lists three cases in which even Beis Hillel agree that they are not shikhechah: "rashei shuros," "the sheaf whose counterpart calls attention to it," and "the sheaf that he took hold of in order to carry it to the city." We explained the sheaf whose counterpart calls attention to it - that the row running in the other direction reminds the person of the sheaf he skipped over; and we saw that our mishnah fits specifically according to the version of Rabbi Yehoshua, in which Beis Hillel did not disagree in a case where he took hold of the sheaf, whereas according to the version of Rabbi Eliezer, in which Beis Hillel consider even taking hold of it to be shikhechah, their words do not fit with our mishnah.
In the next mishnah we will deal with the explanation of "rashei shuros" and the details of their definitions.