Before us is Mishnah 11 in the sixth chapter of Maseches Pe'ah. The Mishnah discusses three cases in which the forgetting is caused by an external factor, and then two cases in which the harvester's intention affects whether the law of shich'chah (forgotten produce) takes effect.
The text of the Mishnah:
"Hakotzer balaylah, v'hame'amer, v'hasuma - yesh lahem shich'chah" - one who harvests at night, one who binds sheaves, and a blind person: the law of shich'chah applies to them. Three cases are presented here:
"Hakotzer balaylah" - one who harvests at night and forgot to cut part of the grain. This is shich'chah of standing grain, produce still rooted in the field.
"V'hame'amer" - one who binds sheaves of grain at night and forgot a sheaf.
"V'hasuma" - a blind person who harvests or binds sheaves, whether by day or by night, and because of his blindness forgot standing grain or a sheaf.
In all of these cases the Mishnah rules "yesh lahem shich'chah" - the law of shich'chah applies.
At first glance one might have argued otherwise: the forgetting here is caused by an external factor, whether the darkness or the blindness, and as we have learned, forgetting that comes about for some other reason is not necessarily considered shich'chah. Nevertheless we do not say so, because the factor liable to cause the forgetting is known to him and he is aware of it in advance. For that reason he certainly took care, while working at night or while working without sight, not to leave anything behind, and therefore the law of shich'chah applies.
"V'im hayah miskaven litol es hagas hagas - ein lo shich'chah": and if he intended to take the largest sheaves first, the law of shich'chah does not apply to him.
This refers to someone whose plan is to gather only the large sheaves first and to leave the small ones behind, and in the course of doing so he forgot even some of the large sheaves. In this case the law of shich'chah does not apply at all, not even to the large sheaves he forgot. There are two reasons for this:
Regarding the small sheaves: since he deliberately left them behind, shich'chah certainly does not apply to them.
Regarding the large sheaves: since he plans to come back and gather the small ones, he knows that at that time he will also collect the large ones he forgot, and so there is no forgetting here at all.
"Im amar: Harei ani kotzer al menas she'mah she'ani shocheiach ani etol - yesh lo shich'chah": if he said, "I am harvesting on condition that whatever I forget I will take," the law of shich'chah applies to him.
This is a person who harvests and stipulates a condition that even what he forgets will remain his to take. The Mishnah rules that the law of shich'chah applies and the condition is of no effect. The reason is the well known principle: anyone who makes a condition contrary to what is written in the Torah, his condition is void. The Torah established that what is forgotten belongs to the poor, and a person has no power to harvest while stipulating that he will take whatever he forgets. Such a condition would nullify a law of the Torah, and therefore it has no validity whatsoever, and the forgotten produce belongs to the poor.
In summary: in this Mishnah we learned that one who harvests at night, one who binds sheaves, and a blind person are all subject to the law of shich'chah, even though the forgetting was caused by an external factor, because they were aware of that factor in advance and took care accordingly. One who intends to take the largest sheaves first is not subject to shich'chah at all, neither for the small sheaves left behind deliberately nor for the large ones, since he plans to return and gather them. And one who stipulates that he will take whatever he forgets, his condition is void under the rule of making a condition contrary to what is written in the Torah, and the law of shich'chah applies.