The Magen Avraham (Orah Haim 170:19) uses this part of the Shulhan Aruch to discuss different foods or practices which are known to promote forgetfulness and are therefore to be avoided. Although not formally codified in the Shulhan Aruch, this concept has its basis in different places in the Gemara. For example, the Gemara (Menahot 99b) says that one who willingly forgets the Torah that one has learned is in violation of the Torah violation of “Pen Yasuru” (Devarim 4:9).
The Gemara (Horayot 13b) mentions that eating olives promotes forgetfulness, while olive oil is conducive to good memory. As such, some rabbis say that ifone were to eat olives, adding olive oil would be advisable. Rabbi Haim Kanievsky (Sefer Zikaron) is skeptical of this opinion since the Gemara only says that olive oil helps one’s memory, not that it nullifies the effects of olives. Rabbi Yitzhak ibn Danan (LeYitzhak Re’ah, vol. 2, Orah Haim §50, [pg.5]) says that since most olives are black, the Gemara’s recommendation to avoid olives only referred to black olives, not green ones. Interestingly, there was a custom in Morocco not to eat black olives during the month of Nissan since it is the month in which one should remember the Exodus from Egypt.
Other things which promote forgetfulness include not washing one’s hands when necessary (such as after touching one’s shoe or cutting one’s nails) (Shulhan Aruch 4:18); sleeping with one’s shoes (Yoma 78b); putting two articles of clothing simultaneously (Magen Avraham, O.H. 2:3); leaving open the book from which one is learning and drying one’s hands on one’s clothing. (Gemara).
Summary: One should avoid if possible those things which promoteforgetfulness.