Gemara

If one forgot to bring a knife [to the Azarah] before Shabbos [that is Erev Pesach], he inserts the knife in the wool of a lamb, or in the horns of a goat.

Question: This is Mechamer (causing animals to do Melachah)!

Answer: This is carrying k'Leacher Yad (unskillfully).

Question: It is forbidden mid'Rabanan!

Answer: One may transgress Shevus unskillfully for the sake of a Mitzvah.

Beitzah 35b (Mishnah): One may cover Peros with Kelim to save them from a drip, and one may put a Kli under a drip on Shabbos.

36a (Ula): One may cover even bricks.

(R. Yitzchak): One may cover only Peros Re'uyim (one may benefit from them today);

This is like R.Yitzchak taught elsewhere, that one may move a Kli only for the sake of Heter (something that one may move on Shabbos).

Question (Mishnah): One may put a Kli under a drip on Shabbos.

Answer: This refers to a drip of Ra'uy (usable) water.

36b (Beraisa): If the Kli became full, he may spill it out and return it. He does not refrain.

A drip was ruining Abaye's mill. He asked about Rabah what to do.

Rabah: Move your bed there, to make it (the mill) like Geraf Shel Re'i (a Kli for excrement). Then, you may remove it [since it repulses you].

Question (Abaye): One may not make Geraf Shel Re'i l'Chatchilah!

While Abaye was asking, his mill fell.

Abaye: I deserved this loss, for transgressing my Rebbi's words!

Shabbos 43a (Rav Yosef): Rav Chisda forbids putting a Kli to catch an egg that will be laid because this is Mevatel Kli me'Heichano (makes a Kli forbidden to move).

Question (Mishnah): One may put a Kli under a drip.

Answer: The case is, the water is Ra'uy.

154b (Rav Huna): If an animal was carrying glass [Muktzeh] Kelim, he puts pillows and blankets underneath before untying the ropes to make the sacks fall.

Question: He is Mevatel Kli me'Heichano!

Answer: He discusses small loads.

R. Gamliel's donkey was loaded with [Muktzeh] honey. He did not want to unload it until Motzei Shabbos. It died on Motzei Shabbos.

Question: He could have untied the ropes, and put pillows underneath!

Answer: They would have gotten dirty. Also, one may not be Mevatel Kli me'Heichano.

Question: Leaving the sacks on is Tza'ar Ba'alei Chayim!

Answer: He forbids Tza'ar Ba'alei Chayim only mid'Rabanan.

Kesuvos 60a (Beraisa - R. Marinus): A moaning child may suckle (from a goat's udder) on Shabbos.

Suckling is Mefarek (a derivative Melachah of Dash, i.e. extracting a food) k'Le'acher Yad (done unskillfully). Chachamim did not forbid it to one who is in pain.

(Beraisa - Nachum Ish Galya): If a gutter pipe became clogged [on Shabbos and the water is dripping into the house], one may covertly squash the clogging matter with his foot.

He fixes a Kli k'Le'acher Yad. Chachamim did not decree when this will cause a loss.

Rishonim

Rambam (Hilchos Shabbos 25:24): One may put a Kli under a drip. If the Kli became full, one may spill it out and return it. The water must be Ra'uy for bathing. If not, one may not put a Kli. If he did, he may move it with the repulsive water inside. L'Chatchilah one may not make Geraf Shel Re'i.

Rebuttal (Ra'avad): Why does the Rambam mention improper water? The Gemara permits putting a Kli under a Ra'uy drip, and when it is full he spills it out and returns it, lest the house get dirty, and it will be Geraf Shel Re'i. If no one lives in the house, and there is no concern if it gets dirty, if he is concerned for damage to the house, he puts his bed there so the dirt will be Geraf Shel Re'i and he may spill it, like the case of Abaye's mill. If there is no loss one may not put his bed there, for one may not make Geraf Shel Re'i.

Ramban (Shabbos 154b DH Ha): Rashi explains that the Gemara asked 'there is pain to animals', i.e. this is mid'Oraisa, so it should override Mevatel Kli me'Heichano, which is mid'Rabanan. I disagree. Torah laws do not override mid'Rabanan laws without need! Perhaps we suggested that one unties the ropes, and the bags fall. Even though the Kelim will break, this is better than pain to animals or Mevatel Kli. The Gemara challenged Rav Huna due to Mevatel Kli, and did not answer that we permit to avoid a loss or due to Tza'ar Ba'alei Chayim mid'Oraisa! The Gemara concluded that he (R. Gamliel) holds that Tza'ar Ba'alei Chayim is mid'Rabanan, and in a situation of a loss Chachamim did not decree.

Avodas ha'Kodesh (Beis Mo'ed 3:8:55): One may put a Kli under a drip even on Shabbos, and all the more so on Yom Tov! This is even a drip of improper water, lest the house get dirty. If the Kli became full, one may spill it out and return it.

Ran (Beitzah 20a): One may put a Kli under a drip of water only if it is Ra'uy. If not, it is forbidden due to Mevatel Kli, i.e. he forbids its use.

Rashi (66b DH she'Yesh): The Shevus [of Mechamer k'Le'acher Yad] is less severe than other Shevusim, since k'Le'acher Yad is not common.

Poskim

Shulchan Aruch (OC 338:8): One may put a Kli under a drip on Shabbos. If it became full, he may spill it out and return it.

Beis Yosef (DH Aval, citing the Tur): Even Rashi, who forbids covering Peros from a drip, permits here. The Mishnah explicitly permits on Shabbos.

Kaf ha'Chayim (58): Presumably we permit only for the sake of the house or if he needs the water, but not for the sake of the Kli, e.g. it is a wooden Kli and he does not want its crevices to dry.

Tzitz Eliezer (2:3:3): If an animal fell into a water channel, if one cannot feed it there, he puts pillows and blankets under it, even though this is Mevatel Kli me'Heichano. We cannot learn to other places. We do not permit to lift the animal from there! (Shulchan Aruch ha'Rav permits in order to avoid a big loss.)

Tzitz Eliezer (5): The Ramban holds that the Isur to milk is only mid'Rabanan. Even so, he does not permit due to Tza'ar Ba'alei Chayim. He permits only Mevatel Kli, which is not a total Shevus, and only if there is also a loss. Chachamim decreed [Shevus] even if this is Mevatel a Mitzvah. K'Leacher Yad is different. The Gemara asked why one may carry the knife through the animal. This is Mechamer! It answered that it is Mechamer k'Le'acher Yad. We asked that this is forbidden mid'Rabanan, and answered that we permit Shevus k'Le'acher Yad for the sake of a Mitzvah. Rashi explains that k'Le'acher Yad is less severe than other Shevus. Even the Ramban would permit milking, since it is Shevus k'Le'acher Yad.

Shulchan Aruch (ibid.): This is if the water is proper for bathing. If not, one may not, for l'Chatchilah one may not make Geraf Shel Re'i.

Beis Yosef (DH u'Mah): The Tur asked that R. Yitzchak permits only if it is proper, but the Halachah does not follow him! We permit even if the water is useless! Perhaps the Rambam learns from Shabbos 43a. Rav Chisda argues with R. Yitzchak, and he permits putting a Kli under a drip only if the water is Ra'uy. This is like the Ran. However, the Rambam connotes that he forbids because one may not make Geraf Shel Re'i l'Chatchilah.

Magen Avraham (12): The primary reason is because Ein Mevatlin Kli. However, if it were permitted to make Geraf Shel Re'i l'Chatchilah, it would be permitted, for then one could spill it out.

Gra (DH Mishum): The Rambam's words forced the Shulchan Aruch to say that it is because one may not make Geraf Shel Re'i l'Chatchilah.

Mishnah Berurah (30): Rain is not considered Muktzeh or Nolad. One may drink or bathe with it. The same applies if an animal could drink it.

Mishnah Berurah (31): One may not put a Kli there even if he will consent to leave it there and not move it.

Bi'ur Halachah (DH Asur): Since one may not make Geraf Shel Re'i l'Chatchilah, he must leave it there, and this is Mevatel Kli. The Chayei Adam says that for a great need, one may rely on the Tur who permits even if it is not Ra'uy. Our custom is to remove the water one washed with after waking in the morning, even though it is improper for bathing or for an animal to drink, and also Mayim Acharonim. We must hold like the Tur. Further, this is called Ra'uy, since there is no Isur on the water, only Ru'ach Ra'ah. Not Ra'uy is only if it dripped from the roof and mixed with mud, or there is an Isur at least mid'Rabanan on it, e.g. Tevel.

Kaf ha'Chayim (59): The Acharonim rule like the Rambam and Shulchan Aruch.

Kaf ha'Chayim (63): It is permitted to avoid a loss.

Shulchan Aruch (ibid.): If one put a Kli there, he may move it with the repulsive water inside.

Beis Yosef (DH Im): Since one may move it with the dirty water in it, what forced us to say that the water is Ra'uy? In any case one may move it, so this is not Mevatel Kli! Indeed, we could have said so. We answered according to the questioner, who considers this Mevatel Kli. Alternatively, the Mishnah permits Stam, which connotes even where one may not move Geraf Shel Re'i, e.g. there are no people there. Then, it is permitted only if the water is Ra'uy.

Magen Avraham (13): It is permitted only in a place where it bothers him there.

Gra (DH v'Im): See 308:36. (One may not make Geraf Shel Re'i l'Chatchilah, but if one did, he may remove it.)

Mishnah Berurah (32): One may spill it even before it is full.

Mishnah Berurah (33): He may move it to spill it only if it is in a place where he normally sits, and it repulses him.