Gemara

(Mishnah): Young Kohanim guarded in upper stories in Beis Avtinas and Beis ha'Nitzutz. They slept on the floor.

They would not sleep in Bigdei Kehunah. They would fold them under their heads and sleep in Chulin garments.

27a - Inference: One may benefit from Bigdei Kehunah!

Rejection: They did not put the Bigdei Kehunah under their heads, rather, next to their heads.

Support: They could not be under their heads, for the Avnet contains Sha'atnez!

Rejection #1: Perhaps the other Bigdei Kehunah (without Sha'atnez) were under their heads!

Rejection #2 (Rav Ashi): We can say that they were under their heads. One may not put soft Sha'atnez under oneself, but it is permitted if it is hard.

Menachos 99b (Reish Lakish): One who forgets something that he learned transgresses a Lav - "Hishamer Lecha u'Shmor Nafshecha Me'od Pen Tishkach Es ha'Devarim..."

Horayos 13b (Beraisa): Five things make one forget his learning:

Eating from what a mouse ate from, eating the heart of an animal, eating olives frequently, drinking water left over from washing, and washing one foot over the other.

Some add sleeping with his head on his clothing.

Ten things cause difficulty in learning;

Passing under the reins of a camel, and all the more so under a camel itself; passing between two camels; passing between two women; and a woman who passes between two men (causes difficulties for the men); passing where one can smell a carcass; passing under a bridge which has not had water under it for 40 days; eating bread that was not fully baked; eating the froth that accumulates on the spoon used to stir cooking meat; drinking from a stream that passes through a cemetery; and looking at the face of a corpse;

Some add reading what is written on a tombstone.

Pesachim 66a: Once, Erev Pesach occurred on Shabbos, and they did not remember whether Pesach is Docheh Shabbos. They asked Hillel. Hillel taught that it does. They made him the Nasi. He provoked (the Chachamim): your laziness, i.e. not serving Shemayah and Avtalyon, caused that I became Nasi!

Question: If one forgot to bring a knife to the Azarah before Shabbos, how may he bring it on Shabbos?

Hillel: I heard this and forgot it.

(Rav Yehudah): If a Chacham gets haughty, his Chachmah departs. We learn from Hillel.

Nedarim 22b (R. Yirmeyah mi'Difti): Anger makes one forget his learning and increase foolishness. It says "anger will rest in the lap of fools", and "a fool will spread his foolishness".

Rishonim

Shitah Mekubetzes (99b): (The text is) a Chacham who forgets transgresses.

R. Yonah (Avos 3:8): If one forgets his learning, it is as if he is Chayav Misah. It is common for people to forget. He should have reviewed often, and thought about it all day and night, until it could not leave his heart. Since he did not do so, he is liable, for he will come to rule according to his memory, and forbid the permitted and permit the forbidden, and a transgression will result.

Mordechai (Berachos 193): When one must wash his hands (after leaving the bathhouse or cutting nails...), there is no clear proof whether it suffices to clean one's hands with things other than water. We do not give (Safek) answers regarding dangerous things. It is good to be stringent to wash with water.

Kol Bo (118): One who has haughtiness forgets his learning.

Sefer Chasidim (1008): One is liable for forgetting his learning only if he sits and removes [words of Torah] from his heart. For one who engaged the entire day in idle words, it would be better that he eat from what a mouse ate, so he forget those idle words!

Poskim

Shulchan Aruch (OC 4:18): If a Chacham does one of these things (that obligate Netilas Yadayim) and does not wash afterwards, he forgets his learning. If he is not a Chacham, he goes crazy.

Source: Beis Yosef DH Kasav ha'Rav, from Mahari Avuhav citing Orchos Chayim.

Kaf ha'Chayim (Palagi, 8:34,35): The simple meaning is that one who does one of these things and does not wash, he deserves to forget his learning, for Torah is acquired only through Taharah and Kedushah. While he is Tamei, Torah leaves him. In a dream, I saw how Netilas Yadayim enables one's Torah and Mitzvos to water saplings in Hash-m's garden. Therefore, not washing causes one to forget Torah. The same applies to any sin!

Teshuvos v'Hanhagos (1:3, b'Sof): The Chazon Ish said that since the Shulchan Aruch says that if a Chacham does not wash, he forgets his learning, one who is not careful to wash transgresses the Lav "Hishamer Lecha."

Maseches Kalah (20): R. Yehoshua says, children die due to Bitul Torah (of the parents) - "v'Shachachta Toras Elokecha Eshkach Banecha Gam Ani."

R. Akiva Eiger (YD 376:4): Likutim in the end of Ma'aneh Lashon say that reading writing on a grave causes forgetting, like it says in Horayos.

Mishnah Berurah (2:2): Putting clothing under one's head (when sleeping) causes forgetting. Perhaps it is not a problem if something else interrupts between his head and the garment. One should not put on two garments at once, for it causes forgetting.

Yalkut Yosef (2:11,12 and Ha'aros 11,12): According to letter of the law, one may remove two garments together. Also ha'Gaon R. S. Z. Auerbach, Ztz"l permits (v'Alehu Lo Yibol p.48). In any case it is proper not to, lest he come to put on two garments together. Putting garments on together does not allow Or ha'Makif to enter; this leads to forgetting. Therefore, there is no concern about shoes with rubbers on them.

Yabi'a Omer (3 YD 8:5): The Rosh (Sof Tamid 25b) says that they brought pillows, so (even in the Hava Amina that the garment were under their heads) there was a separation between the head and their garments. Also, due to the merit of the Mikdash, there is no concern for forgetting there. One of the things that cause forgetting is washing one foot over the other. R. Yosi b'Rebbi Yehudah holds that this is how they did Kidush in the Mikdash! This shows that there is no concern in the Mikdash. However, perhaps that is different, for it is a Mitzvah to do so. Teshuvas Shem mi'Shimon (YD 17) says that the Mitzvah protects.

Taharas ha'Mayim (Ma'areches Shin, 14): The Gemara forbids putting Bigdei Kehunah under one's head for other reasons, but not due to forgetting. This shows that one who does not want to fulfill what it says in Horayos does not transgress, and we do not protest. However, we could say that the Gemara could have asked that this causes forgetting, but it asked a better question.

Rebuttal (Yabi'a Omer ibid., 6): The Gemara gave another answer, that the clothes really were under their heads!

Mishnah Berurah (2:2): Looking at the face of a Mes or reading what is written on a tombstone causes one to forget his learning.

Kaf ha'Chayim (Sofer 2:3): The Gemara says that passing through a cemetery and looking at the face of a corpse are detrimental to learning. We can say that because he forgets, it harms his learning. The concern is only for writing that sticks out, but not what is engraved into the stone.

Keser Rosh (in Sidur ha'Gra p.565, 67): The concept (Isur) of forgetting one's learning applied in their days, when people learned orally.

Reishis Chachmah (Sha'ar ha'Kedushah 5): If one is Metamei himself a little, they (from Shamayim) are Metamei him a lot. Klipos are prepared to dwell on man. Perhaps Chachamim teach that there are Tum'os not mentioned in the Torah. They are a Kabalah from R. Shimon bar Yochai, e.g. the Tum'ah of one who sleeps at night. The Zohar lists not washing after sleeping at night among Aveiros that one tramples (transgresses mindlessly). The Zohar (Acharei Mos 76a) says that Shlomo learned from Ashmedai (the king of the Shedim) 1450 laws of Tum'ah. One must be careful about all the Tum'os mentioned in the Gemara, e.g. passing between two women, and all the matters that cause one to forget his learning. All these are minor Tum'os that people trample, e.g. delaying a bowel movement.

Igros Moshe (EH 3:18 DH uv'Etzem): It is Asur for also the Kalah to give a ring to the Chasan at the Kidushin, for this causes people to forget the law of Kidushin (that Kidushin is only through him giving to her). Even if no transgression will result, the Lav of forgetting Torah applies.

Yabi'a Omer (ibid., 8): It seems that there is no Isur to do one of the things that causes forgetting. One is liable only if he removes Divrei Torah from his heart. What one does for a need (e.g. eating from what a mouse or cat ate from) is not called removing Divrei Torah from his heart. Rather, it is one who is idle and does not review his learning. R. Yonah says that he is liable, for he will rule incorrectly and a transgression will result. Also doing things that cause forgetting will lead to this! However, the Rashbatz (Magen Avos) says that R. Yonah does not obligate for Shegagah in Hora'ah (ruling). If so, why is he exempt if his learning was too much? A transgression came through him. He should have refrained from ruling! Rather, one is liable for forgetting itself. Torah was with him and guarded him from death, and he forgot it through negligence. Therefore, if there was no negligence, there is no Isur. However, R. Sofer in His'orerus Teshuvah says that one who does something that causes forgetting transgresses the Lav. One is liable only if he removes Divrei Torah from his heart. The same applies to doing something that obligates Netilas Yadayim, and not washing. If he is a Chacham, he forgets his learning. R. Yosef Palagi (in Ru'ach Chayim) says that one is Chayav Misah for this. Divrei Ze'ev says similarly. They overlooked Sefer Chasidim, which says that things that cause forgetting are unlike removing Divrei Torah from his heart.

Salmas Chayim (1:10): Concern for forgetting applies only when his own garment is under his head. This is why the Gemara did not ask regarding Bigdei Kehunah, for the Kohen does not own them. Alternatively, the Gemara holds like the first Tana in Horayos, who did not list this among things that cause forgetting. Also, they did so only when it was their turn to serve, so they did not do so regularly.

Yabi'a Omer (ibid., 6): I do not know his source to say that this is a problem only if he does so regularly. The Gemara said so only regarding eating olives. All the others apply even if one does so only once! Because there are other answers, we cannot prove from here that the concern is only for one's own garments.

Shmiras ha'Guf veha'Nefesh p.96 - Teshuvas Shem mi'Shimon (YD 17) says that we are not lenient for Sefek-Sefeka (two doubts) in these matters, because they are dangerous. Mili d'Chasidusa (2) and Mili d'Avos (5 YD 5:3) says that we are lenient about a Safek. Likutei Mahari'ach (Seder Tefilas R. Nechunya ben Hakanah) says that if a Ben Torah is not careful about this, he transgresses the Lav "Hishamer Lecha." Ha'Gaon R. Moshe Feinstein heard that the author of the Aruch ha'Shulchan said that even though the things that cause forgetting are based on Segulos (they are not based on something physical), perhaps these are included in the Torah Isur of forgetting Torah.