Gemara

(Beraisa #1): If one was ascending a ladder and the rung fell, he is liable.

(Beraisa #2): He is exempt.

Suggestion: Tana #1 considers this Yeridah, and Tana #2 considers it Aliyah!

Rejection #1: Both consider this to be Aliyah. Beraisa #1 obligates for damages (if the victim did not die). Beraisa #2 exempts from Galus.

Rejection #2: Both Beraisos discuss Galus. In Beraisa #1, the rung was wormy (it is Yeridah, for it sags when he steps on it). In Beraisa #2, the rung was strong.

Rejection #3: In both Beraisos it was not wormy. In Beraisa #1, the rung was loose (it sags). In Beraisa #2, it was taut.

Bava Kama 26a (Mishnah): Man is always Mu'ad, whether he damaged Shogeg or Mezid (with or without intent), awake or asleep. If (b'Shogeg) he blinded a man's eye, or broke Kelim, he pays full damage.

Inference: The Mishnah teaches about blinding an eye, similar to breaking Kelim. Just like the four payments do not apply to Kelim, also to blinding (b'Shogeg)!

26b (Chizkiyah): "Petza Tachas Patza (a wound in place of a wound)" obligates for Shogeg like for Mezid, for Ones like for willing damage.

(Rava): If one never knew that there was a stone in his lap, or knew and forgot, and he stood up, and it fell, he pays Nezek, but not the four damages (pain, Ripuy (medical expenses), Sheves (compensation for inability to work at all while sick due to the injury) and Boshes (embarrassment)).

If one intended to throw a stone two Amos and it went four Amos, or he intended to throw four and it went eight, he pays only Nezek.

27a (Rabah): If one fell off a roof in an abnormal wind and damaged, he pays Nezek, but not the other four damages. If he fell in a normal wind, he pays four damages, but is exempt for Boshes (since he did not intend to damage).

Rishonim

The Rif brings the Mishnah and Rava's teachings.

Rambam (Hilchos Chovel u'Mazik 1:12): If one fell off a roof in a normal wind, he pays four damages, but not for Boshes. for he is close to Mezid. If one fell in an abnormal wind, he pays Nezek, but not the four damages.

Magid Mishneh: For absolute negligence, e.g. he fell in a normal wind, but it was unintended, he pays the four damages, but not Boshes. If there was no negligence, but it was not total Ones, he pays only Nezek. This refers to cases like falling in an abnormal wind. If it was total Ones he is exempt.

Rambam (15): If there was a stone in Ploni's lap, whether he never knew about it, or he knew about it and forgot, and he stood up and it fell and damaged, he pays only Nezek. The same applies to if he intended to throw two Amos and it went four and damaged, or he damaged while sleeping.

Rambam (6:4): If one was ascending a ladder and a rung slipped and he fell and damaged, if the ladder was not strong and firm, he is liable. If it was strong and the rung slipped or became wormy he is exempt, for this was b'Yedei Shamayim.

Rosh (2:15): Man is always Mu'ad regarding Nezek. He is liable for the other four only if he was Mezid or Shogeg close to Mezid. If one fell off a roof in an abnormal wind and damaged, he pays only Nezek. If he fell in a normal wind, he pays four damages, for he is close to Mezid for going up to a roof without a wall around it (one can fall from it in a normal wind). He is exempt for Boshes, for he did not intend to embarrass.

Poskim

Shulchan Aruch (CM 378:1): If one damaged another's money, even if he did not benefit from it, he pays full damage, whether he was Shogeg or Anus (Rema - this is only if he was not totally Anus, like will be explained).

SMA (2 and Drishah 1): In Siman 421, the Tur himself obligates one who fell in an abnormal wind! There, we discuss injuring a person. If one fell in a normal wind, he pays four damages. We are one level more lenient about an abnormal wind, so he pays only Nezek. Here we discuss damaging property, to which only Nezek applies. We are more lenient about an abnormal wind, and totally exempt, for it resembles total Ones. This also explains why the Shulchan Aruch mentions an abnormal wind only in Sa'if 2, but not in Sa'if 1, which discusses damage to Kelim). The Beis Yosef did not hold like my answer, for he questioned the Tur; in the Shulchan Aruch he retracted.

Question (Drishah 1): What is the Tur's source for this? And why didn't he say that he argues with the Rambam, who exempts also damage to property even if it was Ones?

Be'er Heitev (3): The Tur exempts in a case of total Ones (e.g. a tornado wind). He obligates for an abnormal wind if it was not total Ones.

Gra (7): There is a printing error in the Tur here. (Really, he holds that one who falls in an abnormal wind pays Nezek, like he wrote in Siman 421.)

Shulchan Aruch (3): If a rung of a ladder slipped out and he fell and damaged, if the ladder was not strong and firm, he is liable. If it was strong and firm and the rung slipped or became wormy he is exempt, for this was b'Yedei Shamayim. The same applies to all similar cases.

Gra (10,11): The Shulchan Aruch rules like all the answers. If the ladder was not strong, he is liable even if it became wormy, for the beginning was negligence and the end was Ones. The final answer of the Gemara could have give said that all agree that the ladder became wormy. It did not, in order to answer even according to the opinion that exempts the beginning was negligence and the end was Ones.

Shulchan Aruch (421:3): Some obligate for pain, Ripuy, and Sheves even without intent, as long as it was not Ones, rather, Shogeg close to Mezid.

SMA (6): We learn from "V'Chi Yerivun..." (if one tried to kill another, and injured him, he pays Sheves and Ripuy). The same applies to pain.

Gra (3): No one argues about this. The Gemara (27a) explicitly says so (about one who fell in a normal wind).

Shulchan Aruch (ibid.): One is liable for Nezek even if it was Ones, for man is always Mu'ad, whether Shogeg or Mezid, awake or asleep.

SMA (7): "Petza Tachas Patza" obligates for Shogeg. It discusses Nezek. We exempt Shogeg as much as we can, i.e. all other payments.

Shulchan Aruch (10): If there was a stone in Ploni's lap, whether he never knew about it, or he knew about it and forgot, and he stood up and it fell and damaged, he pays only Nezek.

SMA (17): Also forgetting about it is like Ones.

Shulchan Aruch (ibid.): The same applies if he intended to throw two Amos and it went four and damaged, or he damaged while sleeping.

Shulchan Aruch (11): If one fell off a roof in a normal wind and injured, he pays the four payments, but not Boshes. If he fell in an abnormal wind, he pays only Nezek.