Gemara

Question: If the plow blade broke, which worker pays?

Answer #1 (Rav Papa): The one holding the plow handle pays (he should have led it straight).

Answer #2 (Rav Shisha brei d'Rav Idi): The one holding the blade pays (he inserted it too deeply in the ground.

The Halachah follows Rav Shisha. If it is known that it was a rocky area, they both pay (either may have caused it to break).

Bava Kama 10a (Rav Sheshes): A case in which one responsible for part of the damage is responsible for all of it is adding bundles (on an animal, or to a fire).

Question: If it would not have gone (or spread) without the last, this is obvious!

Answer: Rather, it would have gone without it.

Objection: If so, he did nothing!

(Beraisa): If five sat on a bench, and a sixth sat on it, and it broke, he is liable.

(Rav Papa): The sixth man was (fat) like Papa bar Aba.

Question: If it would not have broken without him, obviously, he is liable!

Answer #1: Rather, it would have broken without him.

Objection: If so, he did nothing!

Answer #2: Rather, without him, it would have broken in two hours. Because he sat on it, it broke in one hour. The others can say 'had you not sat on it, we would have left before it broke!'

Objection: He can say 'without the five of you, I would not have broken it alone!'

Answer #3: Really, it would not have broken without him. The case is, he did not sit on it. He only pressed on the men sitting on it. One might have thought that his force is unlike (damage by) his body. The Beraisa teaches that this is not so.

Rishonim

Rif (Bava Metzia 49b): In a rocky land both pay, for both were negligent.

The Rif (Bava Kama 4b) brings the Gemara in Bava Kama.

Nimukei Yosef (DH Ihu): Whenever it would not have broken without the last, he is liable and they are exempt. If it would have broken without the last, they are liable and he is exempt. The same applies to all cases of negligence.

Rambam (Hilchos Chovel u'Mazik 6:14): If five people put bundles on an animal, and the last put a bundle on and then it died, if it could walk with the first bundles, and after the last it stopped and did not walk, the last is liable. If from the beginning it could not walk, the last is exempt. If it is not known, all pay equally.

Magid Mishneh: The Rambam's text said 'if it would have gone without the last one, this is obvious (that he is liable)! Rather, it did not go even without his. If so, he did nothing! (He is exempt.) Rashi and the Aruch had a different text. The Rambam's text and Perush are primary.

Rambam (15): If five men sat on a chair, and the last man sat, and it broke, he is liable for making it break sooner. They can say 'had you not pressed on us, we would have risen before it broke.' If they sat down together and it broke, all are liable. The same applies to all similar cases.

Rosh (Bava Kama 1:9): The Halachah follows Rav Sheshes' teaching and Rav Papa's Beraisa.

Hagahos Ashri: If it would not have broken without the last, he is liable. If it would have broken without him, they are all liable, even though it broke sooner due to him. The same applies to bundles. Even though it would have spread without his, they are all liable. The same applies to one who digs a pit 10 Tefachim deep, and another deepened it to 20. They are all liable. However, why should one be liable for adding one stick to a great bonfire? This is like one who digs a pit 10 Tefachim deep, and another deepened it to 11. The latter is exempt.

Rashba (10a DH Ha): Indeed, the last is liable only if his bundle could have spread the fire to where it would damage even without the other bundles.

Poskim

Shulchan Aruch (CM 381:1): If five men sat on a chair, and the last man sat on it, and pressed on them and did not allow them to rise, and it broke, since he made it break sooner, he is liable. They can say 'had you not pressed on us, we would have risen before it broke.' If they sat down together and it broke, they are all liable. The same applies to all similar cases.

Shulchan Aruch (383:4): If five people put bundles on an animal, and after the last put a bundle on it died, if it could walk with the first bundles, and after the last added his it stopped and did not walk, the last is liable. If from the beginning it could not walk, the last is exempt.

SMA (4): The last cannot say 'you should have removed your bundles when I put mine on', like we say about five who sat on a bench. There, even without the last one it would have broken after a while. Therefore, he can say 'since you needed to rise anyway, you should have risen immediately.' This does not apply here, or regarding loading a boat (380:4).

Shulchan Aruch (ibid.): If it is not known, all pay equally.

Gra (7): If five sat on a chair and broke it, all are liable (Tosefta 2:8). If it broke due to the last, he is liable. This implies that Stam, all are liable.

Ketzos ha'Choshen (1): Why do we make them pay mi'Safek? The first ones can say that it is due to the last, and he can say that it is due to all of them! The Rambam learns from Bava Metzia 80a. If we know that it was a rocky area, they both pay. Rashi explains that they should have been very careful. It is a Safek, so both pay. Presumably, the same applies here, when we are unsure who damaged. Tosfos argues, for Sumchus says that we divide money in doubt, but the Halachah follows Chachamim, who say ha'Motzi mi'Chavero Alav ha'Re'ayah. Why do the Tur and Shulchan Aruch rule like the Rambam, against Tosfos? Further, even Rashi could agree here. Surely the Halachah follows Chachamim, but in some places Chachamim decided to divide the money, like Sumchus. We say so only where the Gemara said so. Perhaps we may learn from plowers to all damages. However, Tosfos disagrees. This requires investigation.

Chidushei R. Meir Simchah (Bava Kama 10b): Indeed, if we do not know whether the animal walked after the first put their bundles on, they are exempt, for ha'Motzi mi'Chavero Alav ha'Re'ayah. They are liable when it stood still. They should have removed their bundles, lest it cannot walk with them, and he should not have put his on, lest it could have walked. Since all were negligent, they all pay.