Pe'ah, chapter 7, mishnah 2. This mishnah deals with another case involving an olive tree that is not considered shichechah (forgotten produce).
"Zayis shenimtza omeid bein shalosh shuros shel shnei malbenim u'shechacho - eino shichechah":
Two ways of explaining this situation have been offered:
According to the first explanation: a row of three olive trees, then a rectangular vegetable bed, then another row of three olive trees, then again a vegetable bed, and again three olive trees. The olive tree under discussion is the middle one in the middle row - the most central tree of all.
According to the second explanation: there are no vegetable beds here at all, but rather three rows of olive trees the length of a "malben" - a unit of length that was known in the days of the Mishnah. One row stands on one side of the olive tree, a second row on its other side, and a third row in front of it. It turns out that it is surrounded on three sides by rows of olive trees, and it stands in the center.
The early commentators give two reasons why this is not shichechah:
Because of its location: being the most central tree makes it well known and recognizable, in keeping with the criterion of the previous mishnah - something that is well known because of its central location.
Because it is hidden: the reason it was forgotten is that it is thoroughly concealed by all the olive trees surrounding it. The law of shichechah applies only when a person forgets due to his own forgetfulness, and not when the item is hidden and out of his sight.
The mishnah brings another example: "Zayis sheyeish bo sa'atayim u'shechacho - eino shichechah" - an olive tree that yields two se'ah, which the owner forgot, is not shichechah. We have already mentioned the rule that a sheaf containing two se'ah is not considered shichechah, and the same applies to an olive tree: because of its great size, he will surely remember it.
When do these laws apply?
Both laws - both the middle olive tree and the tree that yields two se'ah - were stated specifically "bizman shelo hischil bo" - as long as he has not yet begun to harvest it. But "hischil bo, afilu k'zayis haNetofah bish'ato u'shechacho - yeish lo shichechah": once he has begun the harvesting process, even the famous Netofah olive tree mentioned in the previous mishnah, at the time when it was famous, if he forgot it, it is shichechah.
Incidentally, we noted that in the previous mishnah the word "afilu" in the phrase "afilu k'zayis haNetofah bish'ato" is out of place, and it is clear that it was copied there by mistake from our mishnah, where it makes perfect sense: here the point really is that even a famous olive tree, once he has begun the harvest, is considered shichechah.
Until when is the owner considered still occupied with the harvest:
The mishnah rules: "kol zman sheyeish lo tachtav - yeish lo b'rosho" - as long as olives remain beneath the tree that have not yet been gathered, the fruit at the top of the tree is not considered shichechah. Since the owner is going to return to those olives beneath the tree, which he did not forget, even if he forgot the fruit at the top, when he returns he will remember it as well.
Rabbi Meir extends the time even to a later stage in the process: as long as the beating stick has not yet been passed over the tree - a stick with which they would beat the branches to knock down the hidden olives - there is no shichechah. Even if no olives remain beneath the tree and he has gathered them all, as long as the beating process has not yet been carried out, the olives that were forgotten are not considered shichechah. But once the beating stick has been used and the owner has moved on, it is shichechah.
In summary: in this mishnah we learned two cases in which a forgotten olive tree is not considered shichechah: an olive tree standing in the center of three rows between two rectangles - whether because of its central location or because it is hidden; and an olive tree that yields two se'ah - because of its size. Both laws apply specifically before he began the harvest, but once he has begun, even the Netofah olive tree in its day becomes shichechah. And there is a dispute as to how long the owner is considered occupied with the harvest: as long as olives remain beneath the tree, the fruit at the top is not shichechah, and according to Rabbi Meir, until the beating stick has been passed over the tree.