Gemara

82b (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 #1 - R. Meir): A Shomer Chinam or a Shomer Sachar swears;

R. Yehudah says, a Shomer Chinam swears. A Shomer Sachar must pay.

Contradiction (Beraisa #2 - R. Meir): If Shimon's jug broke or his camel fell and he didn't clear them from the Reshus ha'Rabim, he is liable for resulting damage;

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. A Shomer Sachar pays even without negligence. 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.

R. Chiya bar Yosef enacted that people who carry on shoulder-poles (one burden in front and one in back) must pay half if the load breaks.

This is because it is more than one person should carry, but it is not enough that people would hire two workers for it. It is in between Ones and negligence.'

One who carries on a cross-poles (it straddles the shoulders, one burden on the right, one on the left) must pay fully if the load breaks.

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. If one carried on a pole (on his shoulder) and the load broke, he pays half. It is too big for one, and too small for two.

Rif: If one carries on a lattice of poles (on his shoulders, and it broke), he pays everything.

Rambam (Hilchos Sechirus 3:2): If Reuven was paid to transport a barrel, and broke it, mid'Oraisa he must pay. This is not a big Ones. It is like theft or loss, for which he is liable. Chachamim enacted that he swear that he was not negligent. If he were obligated to pay, no one would agree to transport a barrel! Therefore, they considered the barrel breaking like an animal that died or was broken. They also enacted that if two carried it on a pole and it broke, they pay half. Since the load is too big for one and light for two, it is between Ones and negligence.

Magid Mishneh: Since the load is improper for two, they should pay fully. They were negligent; each should have carried a load proper for one. Since it was light for two, they should be exempt. Therefore, they pay half.

Rambam (ibid.): They pay half if witnesses say that they were not negligent. You learn from here that if one carried a big barrel that normally movers do not carry it (alone), he was negligent. If it broke, he pays everything.

Rebuttal (Ra'avad): The Rambam holds that the negligence was carrying a heavy barrel. I say that the negligence is because it was light. One who trips while carrying a light load is negligent.

Magid Mishneh: Rashi, the Rashba and other Meforshim explain that one person alone carried the load (which was too much for one and too little for two). Rashi says that it is (somewhat) Ones because normally one does not hire two for it, and one tries to carry it himself. The Rashba says that it is because tripping is like Ones. Whenever the load is too small for two, Rashi and the Rashba obligate half, the Rambam obligates fully, and the Ra'avad totally exempts.

Poskim

Shulchan Aruch (CM 304:1): If Reuven was paid to transport a barrel, and broke it, mid'Oraisa he must pay. This is not a big Ones. It is like theft or loss, for which he is liable. Chachamim enacted that he swear that he was not negligent. If he were obligated to pay, no one would agree to transport a barrel! Therefore, they considered the barrel breaking like an animal that died or was broken.

Beis Yosef (DH ha'Ma'avir): The Tur says that one who trips is negligent unless the ground was inclined. This is how Rashi explains.

SMA (1 and Drishah 1): The Shulchan Aruch is like the Rambam, Rif and Rosh, who do not distinguish between level and inclined land. The Tur holds that one who tripped on level land was Vadai negligent. He should have been more careful. R. Eliezer heard that one is exempt even on level ground; he holds that even on an incline, he was negligent and even a Shomer Chinam should pay. We hold that Chachamim did not enact to exempt one who Vadai was negligent. However, it is a Mitzvah to go beyond the letter of the law. The Rif, Rambam and Rosh learn from R. Elazar that R. Meir and R. Yehudah exempt even on level ground. Surely the Halachah follows them. Why didn't the Beis Yosef mention that the Tur is unlike all Poskim?

Shach (1): R. Yehudah holds that one who trips is Ones, even on level ground. R. Elazar himself disagrees.

SMA (3): Since it is like Ones, the Rambam, Tur and Shulchan Aruch did not need to say that the worker is paid.

Shulchan Aruch (2): They also enacted that if two carried it on a pole and it broke, they pay half. Since the load is too big for one and light for two, it is between Ones and negligence.

SMA (5): The Ir Shushan says that the negligence is that each relies on the other to help. This is wrong. Rather, since it is not normal for two to carry it, it is as if one carries it, and it is too much for one.

Shulchan Aruch (4): You learn from here that if one carried a big barrel that normally movers do not carry it (alone), he was negligent. If it broke, he pays everything.

Rema: All this is from the Rambam. Some disagree and hold that whenever the load is too small for two, even though it is too much for one, he is not totally negligent, and pays only half.