Gemara

There were four Luchos in the Aron, two whole ones and the two broken ones - "Asher Shibarta v'Samtam ba'Aron."

Berachos 8b - R. Yehoshua ben Levi (to his sons): Be careful about [honoring] a Chacham who, due to Ones, forgot his learning;

The Luchos and the broken Luchos were both in the Aron.

Menachos 99a (Rav Yosef): "Asher Shibarta v'Samtam ba'Aron" teaches that the broken Luchos were put in the Aron with the [whole] Luchos;

This teaches that if a Chacham forgot his learning due to Ones, we do not act disgracefully to him.

Avos (Mishnah 3:8): One who forgets something from his Mishnah is Chayav Misah (b'Yedei Shamayim).

Rishonim

R. Bechayei (Shemos 32:16 DH veha'Michtav): We learn that one must honor a Chacham who forgot his learning due to Ones, from the broken Luchos, which were both in the Aron, even though the letters departed from them. We must similarly honor the Chacham, since he was once a container of Torah.

Rashi (Sanhedrin 96a DH Hizaharu): One must be careful about his honor because he forgot due to Ones.

Yad Ramah (96a DH Amar R. Zeira): If he forgot not due to Ones, he is Chayav Misah (b'Yedei Shamayim) - "Hishamer Lecha... Pen Tishkach Es ha'Devarim."

Me'iri (8b DH Zaken): We are commanded to honor him as if he did not forget his learning.

Einayim l'Mishpat (8b DH Hizahru): Avi ha'Ezri (1 Berachos 23 DH v'Zaken) and the Me'iri brought this, but the Rambam and other Poskim omitted it. Perhaps they hold that it is merely Midas Chasidus. The Rambam generally omits such matters.

Magen Avos (3:8): A Chachamim who slackens in his learning is a brother to the destroyer. If his learning overcame him, it was too hard for him and therefore he forgot it, he is like a shepherd who is a Shomer Sachar, and the animal went up a cliff (Bava Metzi'a 93b). He is not Chayav Misah (b'Yedei Shamayim), since he was not negligent. Such a Shomer is exempt from paying. This is Ones, and the Torah exempted him. It is proper to honor him. The Yerushalmi says that we treat him with the Kedushah of the Aron, which housed the broken Luchos.

Terumas ha'Deshen (5): If one is Mevatel a Mitzvah to avoid monetary loss, this is called Ones. One must be careful about a Chacham who forgot his learning due to Ones. Rashi (8b DH Machmas) explains that he was sick, or distracted due to lack of food.

Poskim

Question: If Seforim became Pasul and are cannot be fixed, may one put them in an Aron Kodesh for Kosher Seforim? We may learn from the broken Luchos, which were put in the Aron with the whole Luchos. I reject this, for they had extra Kedushah, for Hash-m Himself made them.

Rejection (Noda bi'Yehudah 1 OC 9): The Gemara learned from here that one must be careful about a Chacham who forgot his learning! However, it is not a proof to begin with, for the Aron was made from the beginning to hold the whole and broken Luchos. Surely, a Tanai (stipulation) from the beginning helps. In any case, this is a proper proof about a Chacham who forgot his learning, for Hash-m was particular to make such a Tanai for the broken Luchos. In practice, when a Pesul is found in a Sefer Torah, they return it to the Aron.

Beis She'arim (OC 65 Hagahos): The Taz (OC 155) said that if something is no longer proper for great Kedushah, we demote it to a lower Kedushah. This is difficult, for if a Chacham forgot his learning, we honor him. We learn from the broken Luchos in the Aron! I answer that the Taz agrees about Kedushah itself, i.e. a Sefer Torah or the Luchos. He argues only about Tashmishei Kedushah, e.g. cloths for the Sefer Torah or an Aron.

Zayis Ra'anan (2:51 DH u'Mah): A Mishnah (Bechoros 28b) says that if a judge erred, the verdict stands One might have thought that this includes an error b'Devar Mishnah (a clear mistake) or b'Shikul ha'Da'as, due to his honor. He would rather pay than be embarrassed that he erred. The Rif prefaced his words with "an error bi'Dvar Mishnah is retracted" to teach that this is not so.

Chasam Sofer (2:2 DH umi'Kol): If once Ploni was a certified Shochet, and now we found that he slaughtered and forgot the laws, we cannot rely on "the majority of those who slaughter are experts", for he slaughtered even though he is not expert! Nekudas ha'Kesef [who is Machshir] requires investigation. A case occurred in which witnesses said that the knife of an old Shochet was nicked. Later, he contradicted them, and it was found that he did not feel a nick. The Pleisi distinguishes between forgetting one's learning, and his sense of feeling. I say that in any case mid'Rabanan we forbid all meat and Kelim [dependent on his Shechitah] from when the witnesses found a nick on his knife.

Chayim b'Yad (5 DH uv'Chen): If a Shochet slaughtered, and after people ate it, it was found to be Terefah, it is proper to check if his hands are skilled to slaughter properly. Since his Chazakah was ruined, we must be concerned lest he forgot his learning.

Minchas Chinuch (69:7): Surely, if one received Semichah and a Pesul occurred that disqualifies him from being a judge, the Semichah he received is Batel.

Aruch ha'Shulchan (YD 1:53): If a Shochet became sick and recovered, and found that he does not know the laws of Shechitah, some forbid everything he slaughtered after falling sick, for it seems that his illness confused him (Keneses ha'Gedolah). This is a great reasoning. We find that [Rav Yosef] fell sick and forgot his learning! Therefore, I say that a Shochet who recovered should check himself afresh, to see if he forgot how to feel [for nicks on the knife], or the laws. I did not see people do so. One must consider this matter.

Mishneh Halachos (3:87): The Mechaber (108:8) and Magen Avraham (11) hold that forgetting is like Ones regarding Tashlumim for Tefilah. The Rema (YD 232:12) brings two opinions regarding vows. Why do we honor a Chacham who forget his learning only due to Ones? I say that the Gemara excludes only one who forgot due to laziness, i.e. not reviewing his learning. Even forgetting due to monetary distraction is considered Ones.

B'Tzel ha'Chachmah (3:20): If one's Rebbi forgot his learning due to Ones, one does not say "Rabbeinu taught to us", even if he truly did, for this pains the Rebbi. We find that R. Eliezer forgot, and R. Akiva wanted to remind him. The Gemara asked why he didn't explicitly tell him. I.e. he should have said "Rabbeinu taught to us", for one must do so when challenging his Rebbi. It answered that this is improper, for it will pain the Rebbi.

Acharonim

Maharsha (Eruvin 54a Chidushei Agados DH Sereifah): It says that a Talmid forgot his learning and was Chayav Sereifah, like we say that one who forgets something from his learning is Chayav Misah (b'Yedei Shamayim). He was pardoned because he served a great Chacham. We say that serving [Chachamim] is greater than learning, for one is prone to forget his learning.

Toras Chayim (Sanhedrin 106b DH Echad): The Gemara says that one angel made Do'eg forget his learning, and another burned his Neshamah. Fire could rule over him only after he forgot, for the fire of Gehinom does not rule over Chachamim (Chagigah 27a).

Shlah (Vayakhel Torah Ohr 15): After Chet ha'Egel, Torah was weakened. If not for the breaking of the Luchos, Torah would not have been forgotten (Eruvin 54a). Chachamim warned to be careful about a Chacham who, due to Ones, forgot his learning, for the Luchos and the broken Luchos were both in the Aron. I.e. do not blame him for forgetting, since the breaking of the Luchos caused this.

Sefas Emes (Balak 5660 DH b'Yud Zayen): The Ari Zal explains that in the future, the 17th of Tamuz will be a festival, when Bnei Yisrael will fix the Chet (ha'Egel) and the first Luchos will illuminate. This day is Simchah due to the giving of the Luchos, and sadness because they were broken. Had Yisrael received the first Luchos, there would have been no forgetting of Torah. "Charus Al ha'Luchos" means engraved in the souls of Bnei Yisrael, in a way that cannot be forgotten. Only after the Chet "Achen k'Adam Temusun", they fell to the situation of forgetting. Even so, an impression of the first Luchos remains in the heart. This is why one must be careful about a Chacham who forgot his learning, because the broken Luchos are in the Aron. Shabbos is a semblance of the world to come. It has an influence of the first Luchos. This is the aspect of "Zachor". It is a day of remembrance. The extra Neshamah is the one that went above. The letters [came off the first Luchos, and] were floating in the air. A Neshamah in a body is like Hash-m's name on a parchment.

Kol Mevaser (2:28 DH v'Af): "Korei" always refers to written Torah. "Shoneh" and "Mishnah" refer to oral Torah, which must be repeated. Avos 3:8 discusses one who forgets something from his Mishnah. We say that a Chacham who forgets something from his Talmud transgresses a Lav (Menachos 99b), i.e. only for Talmud.