Gemara

107a (Mishnah): If one pierces an abscess to let out the pus, he is exempt.

Question: What is the source that it is permitted?

Answer (Mishnah): One may move a hand needle, for it is useful to remove a thorn. (Also here, a Melachah she'Einah Tzerichah l'Gufah is permitted to alleviate pain.)

107b (Rav Yehudah): The Mishnah is R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah.

75a (Beraisa): If one hunts a Chilazon and bruises (or presses) it with his hands to extract its blood, he is liable only once.

Question: He should be liable also for killing! Abaye and Rava both say that R. Shimon agrees about a Pesik Reishei (an inevitable result, for it is as if he intended)!

Answer: He prefers that it stay alive, for then the blood makes a clearer dye.

103a (Beraisa): If one detached endives or pruned [moist, edible] reeds:

If it was for people to eat, the Shi'ur is k'Grogeres.

If it was to improve [the ground], he is liable for any amount.

Question: Also in the Reisha, he improves the ground! Even if he does not intend, it is a Pesik Reishei!

Answer: It was in a Chaver's (another's) field.

111a (Rav Simi bar Chiya citing Rav): One may not wrap a rag tightly in the spout of a barrel on Yom Tov [lest he squeeze out liquid].

R. Shimon agrees about a Pesik Reishei.

116b (Beraisa - Chachamim): If Erev Pesach is on Shabbos, the Korban Pesach is flayed entirely on Shabbos.

(Rav Chisda): They hold that if we may save [from a fire] the case of a Sefer with the Sefer, all the more so we can flay Pesach entirely!

Question: We cannot learn from mere Tiltul (mid'Rabanan). Flaying is a Melachah!

Answer (Mar bar Rav Ashi): The case is, he does not need the skin.

Question: R. Shimon agrees about a Pesik Reishei!

Answer: He flays thin strips.

120b (Beraisa): If a lamp is in back of a door, one may open and close it normally. If the lamp extinguishes, this is no problem.

Rav cursed those who ruled according to this Beraisa, for even R. Shimon forbids a Pesik Reishei.

Sukah 33b (Beraisa): One may not diminish the berries [on a Hadas branch, which is Machshir it for the Mitzvah] on Yom Tov;

R. Elazar b'Rebbi Shimon permits.

Question: This is like fixing a Keli on Yom Tov!

Answer (Rav Ashi): The case is, he took the berries to eat. R. Elazar holds like his father, who permits Davar she'Eino Miskaven!

Question: R. Shimon agrees about a Pesik Reishei!

Answer: The case is, he had another Hadas.

Rishonim

Rosh (12:1) and Tosfos (103a DH Lo): The Aruch permits l'Chatchilah a Pesik Reishei d'Lo Nicha Lei (he does not want the result), e.g. he does so in another's field, and he does not love the owner. There is not even an Isur mid'Rabanan. If a barrel of wine was corked with flax, Rebbi permitted to remove the cork to take wine. Even though it is a Pesik Reishei that he will squeeze the flax, since the wine falls to the ground and he does not benefit from this, it is permitted. The Aruch's proof is invalid. Here we exempt in another's field, but even R. Shimon forbids!

Rosh: The Aruch brought a proof from the Heter to flay Korban Pesach in thin strips. R. Shimon permits, since he does not need them so much. This is not a proof. Rashi explained that this is not the way of flaying. Alternatively, thin strips too small for an amulet are not considered hide. He brought another proof from R. Elazar, who permits diminishing the berries if he has another Hadas. It is a Pesik Reishei, but since he does not benefit, it is permitted. I reject this. Perhaps Mitzvos are different. Alternatively, R. Shimon permits Davar she'Eino Miskaven. Perhaps he will not need this Hadas, so he did not fix anything. (Even if he will need it, at the time he did not intend for it.)

Tosfos (ibid.): Do not say that since he has another, he does not fix anything, for we needed to say that R. Elazar holds like his father, who permits Davar she'Eino Miskaven! However, we can say that a small improvement like this is forbidden only mid'Rabanan. Chachamim did not decree when he does not benefit.

Rosh and Tosfos: A proof against the Aruch is that we would forbid puncturing an abscess to remove the pus, if not to avoid bodily pain, even though he does not benefit from Binyan of the opening, and he does not intend for it. He intends merely to puncture. Also, we permit using a needle to remove a thorn (splinter) according to R. Shimon, who exempts Melachah she'Einah Tzerichah l'Gufah, due to pain. Even though he does not benefit from the wound, it would be forbidden if not for pain.

Tosfos (103a DH b'Ar'a): In another's land it is Melachah she'Einah Tzerichah l'Gufah.

Tosfos (75a DH Tfei): R. Shimon agrees about a Pesik Reishei in a matter that is Nicha Lei, but not if he does not care. Rashi says that R. Shimon agrees when the person does not care. This is wrong. R. Shimon exempts one who pruned another's field, even though he does not care.

Tosfos (Sukah 33b DH Modeh): When one has another Hadas he need not fix this, so even if he fixed it, it is Melachah she'Einah Tzerichah l'Gufah. R. Shimon normally forbids this mid'Rabanan; for the sake of the Mitzvah he permits here. Rashi (DH Lo) says that when he does not need this Hadas he did not fix anything.

Hagahos Maimoniyos (Hilchos Shabbos 8:2): The Aruch permits a Pesik Reishei d'Lo Nicha Lei. If so, one may eat in another's garden, for he does not benefit from the growth. Shabbos 103a supports this. Sefer ha'Terumah rejects the proof. Even though he is exempt, it is forbidden! I bring a proof for the Aruch from 75a.

Ran (41a DH Aval): The Aruch says that the rag is not in the barrel's wall. If it were, R. Shimon would permit even a Pesik Reishei, since he does not benefit from the wine squeezed out. Rather, it is on top, so what is squeezed out returns to the barrel. Abaye taught that we may put hot metal to blunt the chill of the Kohen Gadol's Mikveh on Yom Kipur, even if it will surely be Metzaref (solidify the metal), since he does not intend for this.

Rebuttal (Ran DH Aval): These are not proofs. Abaye permitted a Pesik Reishei of Tziruf before he heard that R. Shimon agrees about a Pesik Reishei. Here, squeezing is forbidden due to Dash. If the wine is lost, this is unlike Dash.

Poskim

Shulchan Aruch (OC 320:18): If a barrel was corked with flax plugging the hole in the side from which we draw wine, some permit [to remove and re-insert it] if there is no Kli underneath [to catch what is squeezed out]. Even though he will surely squeeze, this is a Pesik Reishei d'Lo Nicha Lei, and it is permitted. Even though they argued with him and forbade, the custom is to be lenient. Some say that this is like a sponge with a handle, which we permit.

Beis Yosef (DH uv'Reish): The Mordechai brought that the Ri forbids even if the wine is lost. Semag and Sefer ha'Terumah favor the stringent opinion. The Magid Mishneh (Hilchos Shabbos 1:6) did not decide. R. Yerucham says that the Meforshim other than the Aruch are stringent; the custom is to be lenient.

Magen Avraham (23): Even according to the opinion that forbids Pesik Reishei d'Lo Nicha Lei, squeezing is a Toladah of Dash. It does not apply when the liquid is lost. The Ran says that in any case, it is forbidden mid'Rabanan.

Gra (DH Yesh): Tosfos and the Rosh refuted the Aruch.

Bi'ur Halachah (DH Yesh): it seems that the Shulchan Aruch holds like those who forbid Pesik Reishei d'Lo Nicha Lei mid'Rabanan. In this case he condones the custom to be lenient, for it is like a sponge with a handle. Here there is another reason to be lenient, for it is unlike Dash. Some Poskim hold that Libun (laundering) applies with liquids other than water. If so, one can be liable for Libun for plugging up the hole. This is Nicha Lei, even if the wine is lost!

Kaf ha'Chayim (97): One may not squeeze water from a garment, even if the wine is lost, due to Libun.

Kaf ha'Chayim (99): Regarding Shabbos, the Torah forbids only Meleches Machsheves. For other Isurim, even the Aruch forbids Pesik Reishei d'Lo Nicha Lei (Rosh, Magen Avraham).

Bi'ur Halachah (DH d'Lo): Here, he is unhappy that wine is squeezed and lost. Even when one does not care about the Pesik Reishei, also this is called Lo Nicha Lei, like Tosfos says.

Shulchan Aruch (OC 336:3): Those who eat in gardens may not wash their hands over vegetation, for this waters them. Even though they do not intend for this, it is a Pesik Reishei.

Beis Yosef (DH Aval ha'Ochlim): The Aruch permits in another's garden, but surely those who argue with the Aruch forbid.

Magen Avraham (6): Sefer ha'Zichronos says that the lenient opinion permits only in a Nochri's garden. Then, he is not pleased with the growth. The Gemara connotes unlike this; it says his 'Chaver's (friend's) garden. See what I wrote in 307:7. (In one answer, the Ran says that we are more stringent about extinguishing, for when it is Melachah she'Tzerichah l'Gufah, one is liable for it.)

Mishnah Berurah (27): Some permit a Pesik Reishei d'Lo Nicha Lei l'Chatchilah, so it is permitted in a garden that does not belong to him and not to someone he likes. Many forbid Pesik Reishei d'Lo Nicha Lei l'Chatchilah.

Kaf ha'Chayim (26): Mateh Yehudah (or Mateh Yosef) says that the Shulchan Aruch holds like those who forbid a Pesik Reishei d'Lo Nicha Lei.

Shulchan Aruch (OC 646:2): One may not diminish the berries on Yom Tov.

Mishnah Berurah (8): It is permitted only for someone else to eat the berries, or the owner himself if he has other Hadasim. Then it is a Pesik Reishei d'Lo Nicha Lei.

Shulchan Aruch (277:3): If a lamp with oil was on a tabletop, one may not shake it off, for he will surely make the oil go close to the wick, which is burning.

Bi'ur Halachah (DH Iy): Really, the Shulchan Aruch should have said "go close to or distance from the wick", like the Rosh brought from the Yerushalmi. The extinguishing is a Pesik Reishei d'Lo Nicha Lei, for the oil spills and is wasted. All agree that he is exempt, but in any case most Poskim forbid. (In Sa'if 1, when the lamp is in the door, it is a Pesik Reishei d'Nicha Lei, for he retains the oil after it extinguishes.)

Mishnah Berurah (17): One may not shake the table, but he may move it gently, for then it is not a Pesik Reishei.

Rema (316:3): One may not close a small box in which there are flies, for it is a Pesik Reishei that they will be trapped there. Some are lenient if when he opens the box to take them, they will flee.

Bi'ur Halachah (DH v'Yesh): The Pri Megadim (9) is more lenient if there is food in the box, for then he is upset if flies are there. We are more lenient about a Pesik Reishei d'Lo Nicha Lei (that upsets him).

Tzitz Eliezer (6:34:4): For the sake of a Mitzvah, we permit a Pesik Reishei that one does not want. (E.g. one puts ice in an empty bowl, so he will have ice to put in his drink, and some ice will melt in the bowl.) Oneg Shabbos is considered a Mitzvah.