Gemara

(Mishnah): If Reuven was transporting a barrel, and broke it, whether he was a Shomer Chinam or a Shomer Sachar, he swears (and is exempt);

R. Elazar: I heard that both are exempt. It is a wonder!

(Beraisa - R. Yehudah): A Shomer Chinam swears. A Shomer Sachar must pay.

Contradiction (Beraisa - R. Meir): If Shimon's jug broke or his camel fell and he did not clear them from the Reshus ha'Rabim, he is liable for damage that results;

Chachamim exempt b'Yedei Adam, but obligate b'Yedei Shamayim.

We hold that they argue about whether or not tripping is called negligence!

Answer #1 (R. Elazar (the Amora)): Tana'im argue about R. Meir's opinion.

R. Yehudah holds that tripping is not negligence. Therefore, a Shomer Chinam is exempt, but a Shomer Sachar pays even if he was not negligent.

R. Elazar (the Tana) heard that the Halachah follows R. Meir, and questioned this. A Shomer Sachar pays even if he was not negligent! And even a Shomer Chinam swears only if he fell on an incline. Falling on level ground is negligence!

Answer #2 (R. Chiya bar Aba): (R. Meir holds that one who trips is negligent.) Chachamim enacted that he may swear, lest no one agree to transport a barrel!

83a (Rava): He swears that he did not break it intentionally.

Bava Kama 31a (Mishnah): If two potters were walking one after the other, and one tripped, and the other tripped over him, the first pays the damages.

(R. Yochanan): Our Mishnah is even like Chachamim, who say that one who trips is not negligent, and is exempt. Here he is liable, for he should have gotten up.

(Rava): Reuven pays the damages to Shimon, both bodily damage and property damage. Shimon pays to Levi only bodily damage, but not property damage.

Question: If one who trips is negligent, Shimon should be totally liable! If one who trips is not negligent, Reuven should be totally exempt!

Answer: Reuven is certainly negligent (because he tripped on his own). Shimon is blameless for falling, but is liable for bodily damage, for he should have gotten up. He is exempt for property damage, for he can say 'I did not dig this pit.'

Rishonim

Rif and Rosh (6:19): R. Yochanan said that the oath is an enactment, lest no one agree to transport a barrel. Rava taught that he swears that he didn't break it b'Mezid. Presumably, he holds like R. Meir, that one who trips is negligent, and it is an enactment to swear. Chachamim would require him to swear that he was not negligent! The Halachah follows our Stam Mishnah, and the enactment.

Rambam (Hilchos Nizkei Mamon 13:8): If two potters were walking one after the other, and Reuven tripped and fell, and Shimon tripped on Reuven, if Reuven could have stood, but did not, he is liable for damage to Shimon. Even though he was Anus at the time he fell, he was not Anus afterwards. He could have stood.

Rambam (10): If Reuven tripped and fell, and Shimon tripped on Reuven, and Levi on Shimon, and each of them could have stood, Reuven pays the damages to Shimon's body, whether he was damaged on Reuven's body or Reuven's load. Shimon pays the damages to Levi's body due to Shimon's body. Shimon is exempt for damage due to his load, for Reuven made Shimon fall.

Ra'avad: This is astounding! The Sugya is like R. Yehudah, who says that one who trips is Ones. If so, the first one's load is not his pit. Also the Rif says so. Rather, this is when he was not Mafkir his load. R. Yehudah taught that if he intended to keep the shards he is liable, even if he fell b'Ones. One who is Mafkir Nezakav due to Ones is exempt.

Magid Mishneh: The Rambam is like the Rif. He ruled (Halachah 7) that tripping is Ones, and one who is Mafkir Nezakav due to Ones is exempt.

Lechem Mishneh: The entire Sugya is even like Chachamim, who say that one who trips is Ones. He is negligent for not standing. The Tur says that the Rif considers one's Kelim to be a pit even after he was Mafkir them. This is astounding! One who is Mafkir his Kelim after falling b'Ones is exempt!

Rosh (Bava Kama 3:9): The primary Perush obligates Reuven for damages to the Shimon's body and Kelim. One who trips is negligent, so he is liable like one who damages. He is liable for damage due to his Kelim, even though he was Mafkir them, for he tripped through negligence. Shimon, who tripped on Reuven, is not negligent for falling. Even so, he is liable like one who damaged, for he should have stood or warned Levi. He is exempt for damage due to his shards, for presumably he was Mafkir them. To answer the Beraisa, we explain Rava differently, like Chachamim, who hold that one who trips is Ones. He was Mafkir his property after falling through Ones. Reuven is liable for damage due to his body, for he should have stood or warned. Shimon, who tripped on Reuven, is liable only for damage of his body. He is more Anus than Reuven. This is a poor answer; they are equally negligent for not standing or warning.

Question (Rosh citing Tosfos, according to Hagahos ha'Grif): Here, Rava established the Mishnah like R. Meir, who says that one who trips is negligent. In Bava Metzia we rule like R. Yehudah, who says that one who trips is Ones!

Answer (Rosh citing Tosfos): Rava did not establish the Mishnah like R. Meir because he holds like him. Rather, Rava holds like Shmuel, that every obstacle is a pit. We cannot establish the Mishnah like this, for it obligates Reuven for damage to Shimon's Kelim. Shimon was partially negligent for tripping over Reuven. We join this to his negligence for not standing or warning, and he is like one who damaged. Chachamim hold that tripping is Ones, so Shimon is like a pit. R. Yochanan said that even Chachamim, who say that one who trips is Ones, obligate for not standing.

Poskim

Shulchan Aruch (CM 413:1): If Reuven tripped and fell, and Shimon tripped on Reuven, if Reuven could have stood, he is liable. If not, he is exempt, even though he could have warned Shimon, since he is distracted with himself. Shimon pays damages to Levi's body from Shimon's body, but not due to his load. Shimon can say 'I did not dig this pit (my load)', for Reuven made him fall.

Drishah (1 DH Hinei): The Rif brought Rava's teaching to be the Halachah. We conclude like Rava's initial explanation. However, the Rosh says that Rava established the Mishnah like R. Meir, that one who trips is negligent, therefore he is like one who damages, and his load is like a pit. The Halachah follows Chachamim, that one who trips is Ones. The Rif explains that Chachamim agree that he is negligent for not standing, even though tripping is Ones. Therefore, the Halachah follows Rava's initial explanation. The Tur explains the Rif like this. The Rosh holds that the Rif considers Reuven to be like one who damages, but he is exempt for his load, since one who trips is Ones. His negligence of not standing does not obligate him for his load.