The mystical danger of stepping on egg shells:[1]

One who steps on egg shells endangers himself to receive Tzaraas/leprosy. [Although many of the Talmudic, and even codified warnings of mystical dangers are no longer followed today[2], practically, many people are accustomed to abstaining from stepping on egg shells even today. Thus, egg shells are not to be thrown in areas that people walk. There is a story recorded regarding the Rebbe Maharash, that he once admonished someone to be careful in this matter.[3]]

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[1] Admur Hilchos Shemiras Haguf Vihanefesh 9; Abayey in Pesachim 112b; Shemiras Hanefesh Os 76; Omitted from Michaber/Rama and Nosei Keilim in Y.D. 116, including Kaf Hachaim 116; Vetzaruch Iyun

[2] See Shabbos 129b and Tzemach Tzedek E.H. 11 that when the public becomes accustomed to perform a danger Chazal state “Shomer Pesaim Hashem; See Igros Kodesh 2:144 that when the danger is associated with matters of Segula [unnatural] then only when people beware from doing it does the Segula danger apply, while if people are no longer careful then the danger subsides completely; See Rashal Yam Shel Shlomo Kol Habasar 12 that today the Ruach Raah no longer resides and only applies in Talmudic times, just as we rule regarding Zugos.

[3] Shemuos Usipurim 1:173 tells over that the Rebbe Maharash once passed the inn of Yitzchak Shaul in Lubavitch and noticed egg shells laying in front of the inn. The Rebbe went on to warn Yitzchak Shaul that in the future he is not to throw egg shells in areas that people walk, as it is forbidden to walk over them.