Peah, Chapter 5, Mishnah 1. This Mishnah deals with a halachah connected to the gadish - the pile of grain formed once the harvesting is finished.
"Gadish shelo lukat tachtav":
The Mishnah discusses a case in which the pile of grain was set down on ground from which the leket had not yet been gathered. Leket is the stalks that fall during the harvesting, and here the poor had not yet come to the field and had no opportunity to collect them from the ground. The owner of the field went ahead and made his pile right on top of the area where the leket was lying.
The halachah in such a case: "Kol hanogei'a ba'aretz harei hu shel aniyim" - whatever touches the ground belongs to the poor. This is a penalty that the Sages imposed on the owner for having piled his grain in a place where leket belonging to the poor was lying. He is not permitted to do so, since by doing this he conceals the leket from the poor, prevents them from reaching it, and mixes their portion with his own.
The Yerushalmi adds that the penalty is so severe that even if the leket lying there is barley and the pile he set down is wheat - even though it is obvious that anything that is not barley cannot be leket - the halachah is still that whatever touches the ground belongs to the poor, even wheat that could not possibly be leket.
"Haruach shepizrah es ha'omarim":
The Mishnah continues and discusses a case in which the wind came and scattered the sheaves of grain over the area containing the leket, so that the leket became entirely mixed in with the sheaves. Here there is no room to penalize the owner, since he did nothing intentionally, yet the mixture exists nonetheless. How, then, do we calculate how much of the grain should be given to the poor?
The Mishnah says: "Omdim osah kamah leket hi re'uyah la'asos, venosein la'aniyim" - we estimate how much leket this field would normally produce, and that amount is given to the poor.
Rabban Shimon ben Gamliel disagrees and says: "Nosein la'aniyim bichdei nefilah" - we give the poor the standard measure of what usually falls.
What is the difference between the two opinions?
The Gemara in Bava Metzia explains that kdei nefilah is a fixed measure: one forty-fifth of what was harvested. Based on this, the Gra explains the dispute:
According to the first Tanna: the estimate is made individually, according to the particular field - how much leket a field of this type would normally produce.
According to Rabban Shimon ben Gamliel: we use a single fixed measure of one forty-fifth, regardless of the type of field.
In summary: in this Mishnah we learned two laws concerning leket that became mixed in: a pile deliberately made over the place of the leket - the Sages penalized the owner so that whatever touches the ground belongs to the poor, even a species that could not possibly be leket; whereas sheaves scattered by the wind, without any intent on the owner's part - the poor are given according to an estimate, and the Tannaim disagree whether the estimate is specific to the field or a fixed measure of one forty-fifth.