Gemara

Version #1 - Question: If a pit was covered properly for oxen, but not for camels, and camels passed over and weakened it, then oxen fell in, what is the law?

Question: If camels often go there, he was negligent. If camels do not go there, this is Ones!

Answer: Camels occasionally go there. Is this negligence, for he knew that camels might go there? Or, is it Ones, for there were no camels at the time?

Answer #1 (Mishnah): If he covered it properly and an ox fell in, he is exempt.

Suggestion: Camels occasionally go there. It was covered properly for oxen, but not for camels (and camels passed over and weakened it).

Rejection (R. Yitzchak bar bar Chanah): It was covered properly for oxen and camels, and worms corroded the cover.

Answer #2 (Mishnah): If he covered it improperly and an animal fell in, he is liable.

Suggestion: Camels occasionally go there. It was covered properly for oxen, but not for camels (and camels passed over and weakened it).

Rejection: Really, it wasn't covered properly for camels, and camels are common. It is no Chidush that he is liable. The case was taught for parallel structure.

Version #2 - Question: Clearly, since it wasn't covered properly for camels, and camels sometimes go there, he was negligent! Rather, worms corroded the cover (and an ox fell in). Do we say that Migo (since) he was negligent regarding camels, he is liable also for corrosion?

Answer #1 (Mishnah): If it was covered properly, and an ox fell in, he is exempt.

(R. Yitzchak bar bar Chanah): The case is, worms corroded the cover.

Suggestion: If it was covered properly for oxen and camels, obviously he is exempt! What else could he have done?! Rather, it was covered properly for oxen, but not for camels, and camels often come. We do not say that since he was negligent regarding camels, he is liable also for corrosion.

Rejection: Really, it was covered properly even for camels (and it corroded). The Mishnah teaches that he need not regularly test that the cover is still strong.

Answer #2 (Mishnah): If it was not covered properly and an ox fell in, he is liable.

Suggestion: It was covered properly for oxen, but not for camels, and camels often come. Since he was negligent for camels, he is liable also for corrosion.

Rejection: Really, it was covered properly for oxen but not for camels. Camels often come, they came and weakened the cover, and then an ox fell in. Obviously, he is liable. This case was taught only for parallel structure.

Answer #3 (Beraisa): If a blind ox fell in, he is liable. If a seeing ox fell in during the day, he is exempt. We do not say that since he is liable for a deaf ox, he is liable for a healthy ox. (Similarly, we do not say Migo from camels to corrosion.)

Rishonim

Rif and Rosh (5:12): If a blind ox fell in, he is liable. We do not say that since he was negligent for a blind ox, he is liable for a seeing ox. Likewise, we do not say Migo he was negligent for camels (where they are common), he is liable for corrosion.

Rosh: If it was covered properly and corroded, he is exempt. He need not test regularly that the cover is still strong. If it was covered properly for oxen but not for camels, and camels came and fell in, he is liable, even though camels are not common. He should have been concerned lest they come. If camels weakened it and oxen fell in, if camels are uncommon this is Ones; he is exempt. If they come occasionally he is liable, for he should have been concerned lest they come.

Rambam (Hilchos Nizkei Mamon 12:4): If a pit was covered properly and corroded and an ox fell in, he is exempt, for it says "and he will not cover it." If it was covered, he is exempt. If it was covered properly for oxen but not for camels, and camels came and weakened it and oxen fell in, if camels are not common he is exempt, for it is Ones. If camels come even occasionally he is liable.

Rambam (5): If the cover corroded and an ox fell in, even if camels often come and he was negligent regarding camels, since the ox fell due to corrosion, he is exempt. The same applies to all similar cases.

Poskim

Shulchan Aruch (CM 410:22): If a pit was covered properly, even if it corroded and an ox fell in, he is exempt, for it says "and he will not cover it." If it was covered, he is exempt.

SMA (35): The Chidush is that he need not fill in the pit so no damage can come from it.

Shulchan Aruch (23): If it was covered properly for oxen but not for camels, and camels came and weakened it and oxen fell in, if camels are not common he is exempt, for it is Ones.

Rema: However, if a camel fell in, even if it can see, he is liable. The camel passed over because it saw that it was covered.

Beis Yosef (DH Chisahu (2)): Tosfos says that surely, he was negligent and liable for camels. Even if they are not common at all, one must be concerned lest they come and fall. One need not be concerned lest they come and afterwards oxen would fall in. The Rosh agrees.

Shulchan Aruch (ibid.): If camels come even occasionally, he is liable (for oxen).

Beis Yosef (ibid.): The Rosh rules like Version #2, which obligates if camels come even occasionally.

Shulchan Aruch (24): If the cover corroded and an ox fell in, even if camels often come and he was negligent regarding camels, since the ox fell due to corrosion, he is exempt. The same applies to all similar cases.

Beis Yosef (DH Chisahu (3)): The Gemara learned from the exemption for a seeing ox, even though one is liable for a blind ox. Tosfos challenged this. There he was negligent only for a blind ox. Here he was negligent for a seeing ox! He answers that we discuss a case in which if camels would pass, the cover would fall, so seeing oxen would not go in.

SMA (37): The Mechaber obligates even when the Ones was unrelated to the negligence. The Rema disagrees.

Rema: Some exempt only if the camels would have made the cover fall had they passed, so seeing oxen would not go in. However, if camels would only weaken the cover, and afterwards oxen would fall in if they pass over, he is liable even for corrosion. Since he was negligent for oxen, he is liable even for corrosion.

SMA (38): The Tur explains that whenever there was negligence for the things that was damaged (e.g. seeing oxen), if there is any way, even far-fetched, to attribute the Ones to the negligence, we do so. I.e. had the cover been stronger, it would not have corroded.

Gra (34): In CM 291:9, we hold like the first opinion (one is liable only if we can attribute the Ones to the negligence). The Yam Shel Shlomo (5:38) explained that whenever the Ones was like what was prone to result from the negligence (e.g. this animal would fall in a pit) since he was negligent, he is liable for Ones. It is not due to 'the beginning was negligence and the end was Ones.' That is only when we can attribute the Ones to the negligence.