One who traveled beyond the boundaries of the teĥum, whether knowingly or unknowingly, forfeits his 2,000 amot and may now only move within his four amot (SA 405:1; n. 1 above). Should he need to move his bowels, he may walk to a place where he will be able do so privately. Afterward, he may distance himself from this place enough to avoid the foul smell, so that he may recite prayers and berakhot, but he may not move more than four amot from that spot (SA 406:1).
One who traveled beyond the teĥum knowingly and reached an area enclosed by a wall or eruv is nonetheless limited to his four amot. Even if he is now inside a house, he may not move more than four amot. In contrast, if he traveled beyond the teĥum unknowingly or under duress, he may walk freely within the enclosed area (SA 405:6; BHL s.v. “aval”).
If one traveled beyond the teĥum in order to save a life, the Sages ordained that upon completing his mission, he may walk 2,000 amot in each direction. If this new teĥum overlaps his original teĥum, he may return home, and he retains his original teĥum as though he never left (Eruvin 44b). In certain cases he may even return to his original place regardless of teĥumin, as explained above in 27:10 and n. 12).
If one is traveling on a plane that, due to unforeseen circumstances, lands in an airport on Shabbat, his teĥum Shabbat is established upon his landing, and he may not go farther than 2,000 amot in any direction. Since an airport is generally surrounded by a fence and often contains an area for sleeping, the whole airport is considered his four amot, and he can walk another 2,000 amot beyond it. However, if the airport is not surrounded by a fence, then his mekom shevita is established the moment the plane touches down. If the plane then taxies on the runway for another 2,000 amot, he has gone beyond his teĥum, and he may not move any farther than his four amot. This means he must remain on the plane until Shabbat ends. If the crew or security personnel insist that he leave, or if he needs to leave in order to use the bathroom, he may do so. If he then reaches an enclosed area, he may move around within it, since the only reason he originally traveled beyond his teĥum is that he was forced to do so (SA 405:6). If his flight was for the sake of a mitzva, then even if the plane taxies for a full kilometer and the airport is not fenced in, he may still walk 2,000 amot from the airplane door (SA 248:4; MB ad loc. 32).
One whose boat docked on Shabbat may leave the boat and walk 2,000 amot in each direction. This is because until reaching the port, the boat was more than ten tefaĥim above the ocean floor, so teĥum Shabbat did not apply to it. Only once he sets foot on dry land is his teĥum established. If the port is fenced in, he may walk 2,000 amot beyond the enclosure (SA 404:1; n. 3 above).
One who traveled beyond his teĥum and then returned inside his teĥum unknowingly or due to circumstances beyond his control may still walk within his teĥum (SA 406:1). However, if he traveled beyond the teĥum knowingly, then even if he returned unknowingly, he forfeits his teĥum, though he may still walk throughout the city (SA 405:8).
Just as one may not travel beyond his teĥum on Shabbat, he also may not move his possessions outside the teĥum. If he took fruit beyond the teĥum unknowingly, even though they may not be carried more than four amot, they may be eaten. If he did so knowingly, the fruit may not be eaten (SA 405:9; MB ad loc. 52; see above, ch. 26 n. 6).
If a non-Jew brought fruit from outside the teĥum on Shabbat, as long as he brought them for himself or for another non-Jew, a Jew may eat the fruit. However, one may not carry them more than four amot. If the non-Jew brought the fruit into a home or a site that is enclosed by a fence or an eruv, one may carry the fruit within the enclosed area. In contrast, if the non-Jew brought the fruit for a Jew, that Jew and the members of his household may not eat the fruit until enough time has passed after Shabbat for the fruit to have been brought then (SA 325:8).
The second set of laws relates to benefiting from prohibited actions done on Shabbat, and the intent of the person transporting the fruits determines their status. If he did so knowingly, no one may benefit from his actions, and the fruit may not be eaten. If he brought them unknowingly, then since the prohibition itself is rabbinic, they may be eaten (Pri Megadim; BHL 318:1, s.v. “ha-mevashel”; see Harĥavot 26:4:1). If a non-Jew brought the fruit from outside the teĥum for himself or for another non-Jew, a Jew may eat them; but if he brought them for a Jew, that Jew and his household may not eat the fruit until enough time has elapsed for the fruit to have been brought to them permissibly after Shabbat.
The Sages established that the laws of teĥumin also apply to objects belonging to non-Jews, and such objects acquire a mekom shevita wherever they are when Shabbat began. If it was permitted to carry objects belonging to non-Jews without limit, people might mistakenly come to believe that objects belonging to Jews are also not subject to the laws of teĥum Shabbat. Ownerless items, however, are not subject to teĥumin restrictions (SA 401:1).