The Gemara in Masechet Horiyot (13b) lists a number of activities that can cause a person to forget the Torah he has studied. These activities include eating olives under certain conditions, and sleeping on one's clothing.

The question was raised as to whether one who engages in such activities transgresses the prohibition introduced in this verse, "But be careful and guard your soul lest you forget the things" (Devarim 4:0), which the Mishna (Avot 3:8) interprets as forbidding one to forget his Torah knowledge. Since these activities cause one to forget what he has learned, one who engages in such conduct is perhaps in violation of a Torah prohibition, and, if so, we must therefore ensure to refrain from these activities.

Indeed, the work "Orot Eilim" (p. 59) comments that a person who involves himself in these activities likely transgresses the Torah prohibition against forgetting Torah. Similarly, Rabbi Yosef Palachi, in his work Ru'ach Chayim (vol. 2), writes explicitly that if a person does not wash his hands in situations regarding which the Sages warned that failure to wash one's hands causes him to forget Torah, he violates this Torah prohibition.

However, Chacham Ovadia Yosef, in his work Yabia Omer (9:41), rules differently. He cites a passage from the Sefer Chassidim (by Rabbi Yehuda Ha'chasid, Germany, 1150-1217; listen to audio for precise citation) concerning the case of a person who had a piece of bread, that a mouse had nibbled from. Among the actions mentioned by the Gemara as causing a person to lose his Torah knowledge is eating food from which a mouse had nibbled, and the individual thus approached a Rabbi and inquired whether he may continue eating the bread being that he was very hungry. The Rabbi answered that he may continue eating that piece of bread, because the Torah prohibition against forgetting one's Torah refers only to making a concentrated effort to lose Torah knowledge. This prohibition would not apply to somebody who happens to forget what he learned, even if this results from his involvement in the activities mentioned in the Gemara. The Rabbi added that the person who posed him the question wastes much of his day involved in vain activities, and it is therefore preferable for him to eat the bread that had been touched by a mouse, as this may cause him to forget all the useless knowledge and information he has acquired through his involvement in vanity.

On the basis of this passage of the Sefer Ha'chasidim, Chacham Ovadia rules that one does not violate any prohibition by engaging in activities that the Gemara warns will cause somebody to lose his Torah knowledge, but indeed one should nonetheless refrain from any activity that may cause him to forget his learning.