Examples of Ayin Hara in Halacha:

1. Orach Chaim: Father and son by Aliyah:[1] From the letter of the law, it is permitted to call two relatives to the Torah for an Aliyah one after the other. This applies whether they are brothers or father and son. However, the custom is to avoid doing so due to Ayin Hara.[2] This prohibition applies even if the relatives claim they are not particular about Ayin Hara.[3]
2. Counting for Minyan: One can say the verse[4] of “Ani Berov Chasdecha” to count for a Minyan, as this verse also contains ten words and is Segula against Ayin Hara.[5]
3. Yoreh Deah: Circumcising two babies at same time: When circumcising two babies on the same day, some Poskim[6] rule that one is not to recite a single blessing on behalf of both children and circumcise them simultaneously due to Ayin Hara. Other Poskim[7], however, rule it is permitted to do so. Practically, the custom is like the former opinion.[8]
4. Twins having Mila at same time:[9] In the event that one is circumcising twin boys on the same day, then one is to recite the verse of Hamalach Hagoel in order to avoid the evil eye [i.e. Ayin Hara].
5. Even Haezer: Siblings getting married same day:[10] A parent is to beware not to have a wedding for two of his children on the same day, as such marriages will not be successful.
6. Rav Yehudah Hachassid-Chasan has same name as father of Kallah, or Kallah has same name as mother of Chasan:[11] [Due to Ayin Hara and danger] a man should not marry a woman whose father shares the same name as he [without adding second name].
7. Choshen Mishpat: Not to look into one’s neighbor’s property:[12] It is forbidden to look into another person’s property [even if they are not present there[13]] if there is possibility that the evil eye can damage their property. Thus, for example, one may not stand over the field of his friend to stare at it at the time that its crops have grown full.  

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[1] Michaber 141:6; Kol Bo 20; Mordechai; See Yalkut Dinei Kerias Hatorah p. 1344; See Sefer Shemiras Haguf Vihanefesh [Lerner] 81:11-19

Other opinions: Some Poskim rule it is forbidden from the letter of the law to call two relatives one after the other, being that they are considered invalid witnesses. [Orchos Chaim, brought in Beis Yosef; Ateres Zekeinim 141:3; P”M 141 A”A 8] According to this opinion/reason, all relatives that are invalid to testify for each other are likewise not to be called to the Torah one after the other. [P”M ibid] Practically, we do not rule like this opinion. [Kaf Hachaim 141:25 based on Gr”a who proofs from the Gemara that even Pesulei Eidus may receive Aliyos, and so is the final ruling of Michaber ibid who rules the prohibition is only due to Maaras Ayin.]

[2] Michaber ibid

[3] Hagahos Beis Yosef; Elya Raba 141:8; Birkeiy Yosef 141:7; Yosef Ometz 16:4; M”B 141:19; Shaareiy Efraim 1:6; Derech Hachaim 14; Kitzur SHU”A 23:13; Poskim in Kaf Hachaim 141:29

Other opinions: Some Poskim rule it is permitted for relatives to forgive their rights on this matter. [Noheig Katzon Yosef, brought in Kaf Hachaim ibid; Aruch Hashulchan 141:8] Practically, we do not rule like this opinion. [Kaf Hachaim ibid]

[4] Tehillim 5:8

[5] Sefer Haitim in name of Rav Haiy Gaon; Sefer Pardes of Rashi 99; Sefer Haorah of Rashi 1:56; Rebbe in Toras Menachem ibid

The reason: This verse contains an advantage over the verse of Hoshia Es Amecha, being that it mentions the words “Avo Beisecha” which refers to a Shul

[6] Perisha E.H. 62, brought in Chelkas Mechokeik 62:3, as rules Rama E.H. 62:2 regarding a Chupah

[7] Michaber and Rama Y.D. 265:5; Taz 265, brought in Chelkas Mechokeik 62:3

[8] See Pesakim Uteshuvos 265:18

[9] Sefer Zechira; Shemiras Hanefesh 139; Kaf Hachaim 116:182; Sefer Shemiras Haguf Vihanefesh [Lerner] 151:1; See also Rama E.H. 62:3 and Chelkas Mechokeik 62:3

[10] Rama E.H. 62:2; Tzavah Rebbe Yehuda Hachassid 30; M”B 546:1; Igros Kodesh 8:154; Sefer Haminhagim p. 76; Shulchan Menachem 6:161

A widow: Some Poskim rule that a widow may marry off two of her daughters on the same day. [Machaneh Chaim E.H. 3:33] Other Poskim, rule that even a widow is not to do so. [Sghem Mishimon E.H. 6] See Shemiras Haguf Vihanefesh [Lerner] 179:3

[11] Tzavas Rebbe Yehuda Hachassid 26, 27; Mishnas Chassidim Miseches Chasuna 1:8; Pischeiy Teshuvah Y.D. 116:6; E.H. 2:7; 50:14; Chochmas Adam 123:13; Tzemach Tzedek E.H. 143 and Piskeiy Dinim Y.D. 116 that the Alter Rebbe was very stringent with this Tzavah; Kitzur SHU”A 145:8; Yosef Ometz 37:3; Zechor Leavraham 3:190; Hieshiv Moshe 69; Avnei Tzedek E.H. 4; Yifei Laleiv 4 E.H.  62:11; Peri Sadeh 1:69; Divrei Chaim E.H. 8; Shemiras Hanefesh 86; Kaf Hachaim Y.D. 116:125; Imreiy Yaakov 10 Likkutim 8; Sefer Shemiras Haguf Vihanefesh [Lerner] 169:1-21 footnote 1 in name of many Poskim; All Poskim in coming footnotes who discuss details of this Tzavah and how to circumvent it

[12] Admur Nizkei Mamon 11; Michaber C.M. 378:5; Bava Metiza 107a; Bava Basra 2b; Sefer Shemiras Haguf Vihanefesh [Lerner] 241:2

[13] See Q&A!