Gemara

(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. He is liable, for he should have gotten up.

(Rav Nachman bar Yitzchak): Even if he was unable to get up, he is liable. He should have warned the other.

R. Yochanan disagrees. Since he cannot get up, he is distracted, and he is not obligated to warn.

Question (against R. Yochanan - Mishnah): If Reuven was walking, carrying a beam, and Shimon was walking in back of him, carrying a jug, and the jug broke on the beam, Reuven is exempt. If Reuven stopped, he is liable.

Suggestion: Even though he stopped to adjust the beam, which is normal, he is liable, for he should have warned (even though he was distracted.)

Rejection: No, he stopped to rest (which is abnormal).

Question: If one who stopped to adjust is exempt, why does the Reisha exempt if he warned? It should distinguish between stopping to rest and to adjust!

Answer: The Mishnah teaches that even when he stops to rest, if he warned, he is exempt.

Question (Beraisa): Potters and glassmakers were walking one after another. Reuven (the first) tripped and fell; Shimon tripped on Reuven, and Levi on Shimon. Reuven pays damages to Shimon, and Shimon pays damages to Levi.

Suggestion: They were unable to stand (they are liable for not warning)!

Answer: No, they could have stood.

Question: If one who could not have stood is exempt, why does the Reisha exempt if they warned? It should teach that one who could not stand is exempt!

Answer: The Beraisa teaches a different Chidush. Even when they could have stood, if they warned each other, they are exempt.

(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 (he tripped on his own). Shimon is blameless for falling, but is liable for bodily damage, for he should have stood up.

Rishonim

Rif: R. Yochanan taught that he is liable, for he should have gotten up. Therefore, even Chachamim, who say that one who trips is not negligent, obligate. Because he did not get up, he is not considered Anus, rather, Mezid.

Question (Rosh 3:9): Why does the Rif rule like R. Yochanan? We should follow Rav Nachman bar Yitzchak. He was a Talmid of Rava, and is Basra! Also, the Gemara challenged R. Yochanan, and gave difficult answers. We said that the Beraisa teaches that he is exempt if he warned the one in back of him. This is no Chidush! It would have been better to teach that if he stopped to adjust the load he is exempt even if he spanned the entire width of the road!

Rambam (Hilchos Nizkei Mamon 13:8): If Reuven tripped and fell, and Shimon tripped on Reuven, if Reuven could have stood, but did not, he is liable for damage to Shimon. He was Anus at the time he fell, but not while lying there. He could have stood. If he could not have stood he is exempt, even though he could have warned Shimon, since he is distracted with himself.

Rambam (Hilchos Chovel u'Mazik 6:8): If Reuven was carrying a beam, and Shimon was in back of him, carrying a jug, and the jug broke on the beam, Reuven is exempt. If Reuven stopped to rest from the weight of his load, he is liable. If he said stop', he is exempt. If he stopped to adjust the beam, it is as if he is walking. He is exempt even if he could have warned, for he was distracted.

Poskim

Shulchan Aruch (CM 379:2): If Reuven was carrying a beam, and Shimon was in back of him, carrying a jug, and the jug broke on the beam, Reuven is exempt. If Reuven stopped to rest from the weight of his load, he is liable. If he said stop', he is exempt. If he stopped to adjust the beam, it is as if he is walking. He is exempt even if he could have warned, for he was distracted and could not warn him (Rema - see below 413:1).

Beis Yosef (DH u'Mah she'Chosav Im): Even though the Rosh questioned why the Rif ruled like R. Yochanan, the Tur brought only the Rif's opinion.

SMA (2): Even if he was needed to stop to rest, he is liable, for he should have warned the one in back.

Shulchan Aruch (413:1): If two potters were walking one after the other, and Reuven tripped and fell, and Shimon tripped on Reuven, if Reuven could have stood, he is liable. If he could not have stood, he is exempt, even though he could have warned Shimon, since he is distracted with his own soul.

Beis Yosef (DH ul'Inyan...): The Rif and Rambam rule like R. Yochanan. It is easy to answer the Rosh's questions against this, therefore we follow them.

Gra (1): We rule like R. Yochanan because the Gemara elaborated to explain him. Likewise, when explaining Rava we say that he is liable for not standing.

Rema: Some obligate if he could have warned but did not warn.

SMA (1): This is like the Rosh, who equates this to the Mishnah (32a) about a jug that broke on a beam. He explains the Mishnah to discuss even when he stopped to adjust the load, for he did not warn. The Rif obligates only when he stopped to rest. Why did the Tur and Rema rule like the Rif in Siman 379:2, and here they said that it is an argument? They must hold that one carrying a beam is more distracted with it than a potter who fell, and the Rosh could agree that the Mishnah is only when he stopped to rest.

Note: The Rosh (10) explicitly says that in the Mishnah, he stopped to rest.

Gra (1): This opinion rules like Rav Nachman bar Yitzchak, for he is Basra. The answers for R. Yochanan are poor. The Gemara answered for Rava that he is liable for not standing. It could also have said that he is liable for not warning.

Be'er ha'Golah (3): The Or Zaru'a and Mahari'ach agree with the Rif and Rambam, and the Rosh did not definitively rule like Rav Nachman bar Yitzchak, so it seems that we follow the Rif.

Shulchan Aruch (2): If Reuven tripped, 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.

SMA (3): The opinion the Rema brought above obligates also here even if he could not have stood, but could have warned.