Gemara

(Abaye): The Tzad ha'Shavah in our Mishnah includes a stone, knife or load left on top of the roof. It fell in a common wind and damaged (after coming to rest).

Question: If the owner was Mafkir them, Rav and Shmuel agree that this is a pit;

28a (Mishnah): If Reuven's jug broke in the Reshus ha'Rabim, and Shimon slipped on the water or was damaged by the shards, Reuven is liable;

R. Yehudah says, if he intended, he is liable. If not, he is exempt.

Question: What is the case of intent?

Answer #1 (Rabah): Reuven intended to take the load off his shoulders.

Question (Abaye): If so, R. Meir (the first Tana, who is more stringent) would obligate even in a case of Ones (beyond his control), e.g. the flask dissolved! The Torah exempts for Ones - "you will not punish the Na'arah (who was raped)"!

(Beraisa - R. Meir): If his jug broke and he did not clear away the shards, or his camel fell, and he did not stand it up, he is liable for damage they caused;

Chachamim say, he is exempt b'Yedei Adam (Beis Din does not make him pay), but liable b'Yedei Shamayim (Heaven holds him accountable).

R. Meir admits to Chachamim that if he put bottles on the roof to dry, and they fell in an abnormally strong wind, he is exempt.

Answer #2 (Abaye): They argue about two matters. At the time of falling, R. Meir says that one who trips is negligent; R. Yehudah disagrees. After falling, R. Meir obligates one who is Mafkir Nezakav (damaging things); R. Yehudah exempts.

29b (R. Yochanan): They argue after the time of falling.

He means that Chachamim exempt one who was Mafkir Nezakav here, for it was due to Ones. Normally, they all agree that he is liable.

R. Yochanan and R. Elazar argued about one who is Mafkir Nezakav. One of them obligates, and the other exempts.

The one who exempts must hold like Chachamim (since R. Meir obligates even one who is Mafkir Nezakav after they fell b'Ones!) However, the one who obligates could hold like Chachamim. Chachamim exempt when Nezakav fell b'Ones. They could admit that he is liable when there was no Ones.

(R. Elazar, citing R. Yishmael): One does not own a pit in Reshus ha'Rabim, but the Torah obligates him for them as if he owned it.

Question: R. Yochanan agrees! He said that the Halachah follows a Stam Mishnah, and a Stam Mishnah obligates a pit in the Reshus ha'Rabim!

Answer: Elazar cited R. Yishmael to say that one is liable. He himself exempts.

Rishonim

Rif: Abaye taught that they argue in two cases. At the time of falling, R. Meir says that one who trips is negligent, and R. Yehudah disagrees. After the time of falling, R. Meir obligates one who is Mafkir Nezakav, and Chachamim exempt, unless he intended to acquire them. He is exempt even after falling, since he was Ones at the time he fell, the water and shards are like Hefker that came by themselves. This is the Halachah. R. Yochanan and R. Elazar argue about one who was Mafkir Nezakav not due to Ones. The one who obligates could hold like Chachamim. Perhaps even they obligate when there was no Ones. The Halachah follows R. Yochanan, who obligates.

Rosh (3:4): The Rif holds that Abaye says that they argue about two matters, due to one reason. R. Yehudah exempts Mafkir Nezakavz because he holds that one who trips is Ones. Abaye holds like R. Yochanan. The Ri explains that R. Yehudah exempts even after falling through negligence. The entire Sugya is like him. However, l'Halachah there is no difference. Even though Abaye is Basra, and he holds that R. Yehudah exempts even after falling through negligence, the Halachah does not follow R. Yehudah. This is because there is an argument in the Mishnah here, and later (49b) a Stam Mishnah obligates a pit in Reshus ha'Rabim. Also R. Yochanan holds like this.

Nimukei Yosef (DH ul'Acher): R. Meir holds that even though one was Mafkir the shards, since they were initially his and he should have removed them, if he did not he is liable like one who dug a pit in Reshus ha'Rabim. It is Hefker, and he is liable. R. Yehudah exempt, for he holds that the Torah obligates only for a pit in one's Reshus, when he later was Mafkir his Reshus but not his pit. If he intended to cast down the jug while falling, he is liable, even though tripping is Ones. Even if one may do anything to save himself, it is not better than Reshus. The Halachah follows R. Yochanan against his Talmid R. Elazar; the Halachah follows a Stam Mishnah; the Halachah follows a majority, i.e. Chachamim against R. Meir.

Rambam (Hilchos Nizkei Mamon 13:7): If Reuven's jug broke in the Reshus ha'Rabim, and Shimon slipped on the water or was damaged by the shards, Reuven is exempt b'Yedei Adam, for he was Anus. He is liable b'Yedei Shamayim for not clearing away the shards. The shards and water are like Hefker, but he was not Mafkir until after the Ones. Therefore, he is exempt. If he intended to acquire the shards and another was damaged through them, he is liable. The same applies if his camel fell and he did not stand it up. If Kelim were damaged through these, he is exempt whether or not he was Mafkir.

Magid Mishneh: He rules like R. Yehudah in the Mishnah, and Chachamim in the Beraisa.

Poskim

Shulchan Aruch (CM 412:4): If Reuven was carrying a jug and he tripped and it broke, and one was damaged by the shards after they landed or slipped on the water that spilled from the jug, Reuven is exempt b'Yedei Adam. This is because he was Mafkir Nezakav after falling b'Ones.

SMA (7): Stam, we assume that he was Mafkir the shards of a Kli that broke on the way and the water that spilled to the ground.

Shulchan Aruch (ibid.): He is liable b'Yedei Shamayim if he had time to clear away the shards.

Gra (5): We must say that there was time to clear away the shards. If not, Chachamim, who say that one who trips is Ones, would not obligate b'Yedei Shamayim (Tosfos DH mid'Masnisin).

Shulchan Aruch (ibid.): If he intended to acquire the shards, he is liable for what they damage afterwards, for it is a pit. If he tripped through negligence and the jug broke, he is liable even if he made them Hefker.

Beis Yosef (DH u'Mah she'Chosav v'Im): No one exempts if he fell through negligence.

Gra (6): The Ri says that R. Yehudah exempts even if he fell through negligence. The Halachah does not follow R. Yehudah regarding this. The Shulchan Aruch rules like R. Yehudah because our entire Perek calls him 'Chachamim'.

Shulchan Aruch (ibid.): He is liable b'Yedei Shamayim if he had time to clear away the shards.

SMA (8): Since he tripped through negligence, he is like one who was Mafkir Nezakav after putting them in a place where he was not allowed to put them.

Gra (8): If one put bottles on the roof to dry, and they fell in an abnormally strong wind, he is exempt even if he did not clear them away afterwards. Had they fallen in a normal wind, he would be liable.