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Birchas Hamazon:

Yaaleh Veyavo:^[1]

During Chol Hamoed, one recites Yaaleh Veyavo in Birchas Hamazon. If one forgot to recite it, he does not repeat Birchas Hamazon.

Remembered after the blessing of Uvinei Yerushalayim, but prior to beginning Hatov Vehameitiv:^[2] If he remembered prior to beginning even the first word of the blessing of “Hatov Vehameitiv” then he is to recite the blessing of Baruch Ata Hashem Elokeinu Melech Haolam Asher Nasan Moadim Liamo Yisrael Lisason Ulisimcha Es Yom Chag Hamatzos Hazeh”.^[3]

Remembered after beginning the 4^th blessing:^[4] If one forgot to say Ya’aleh Veyavo in its set area, and he remembered only after he already began the first word “Baruch” of the blessing of Hatov Vehameitiv, then he has fulfilled his obligation, and Birchas Hamazon is not to be repeated. [He is not to recite any extra blessing on behalf of Yom Tov and is rather to continue as usual.] In such a case, it is not customary to recite Ya’aleh Veyavo in the Harachamans.^[5] 

Harachaman and Migdol:

Harachaman: Harachaman of Yom Tov is not recited on Chol Hamoed.

Migdol:^[6] When reciting Birchas Hamazon on Chol Hamoed, one recites “Migdol” as opposed to “Magdil”.

Meiyn Shalosh:

Some Poskim^[7] rule one is to mention Yom Tov within the after blessing of Meiyn Shalosh, and so is the Chabad custom.^[8]

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[1] Admur 490:4

[2] Michaber ibid; Admur 188:10; Siddur ibid

[3] The concluding Nussach: It is questionable as to whether one should conclude this blessing with the words “Baruch Ata Hashem Mikadesh Yisrael Veroshei Chodashim.” Practically, Safek Brachos Lihakel and therefore one should not recite it. [Admur ibid and so rules Admur in his Nussach in Siddur]

[4] Admur 188:9; Siddur ibid; Ketzos Hashulchan 47:3; Tehila Ledavid 188:4

Other opinions: Some Poskim rule one may still recite the added blessing [brought next] so long as he has not yet said the word “Hakeil”. [Chayeh Adam brought in M”B 188:23; In the Biur Halacha he leaves this ruling with a “Tzaruch Iyun Gadol”, however he implies that simply the words Baruch Ata, in his opinion, is not problematic.

[5] Admur 188:12; Rama 188:7

The reason: Although regarding Al Hanissim we rule that if one forgot to mention it in Birchas Haaretz, he is to mention it within the Harachamans, nevertheless regarding Ya’aleh Veyavo the custom is not to do so. The reason for this is because the prayer of Ya’aleh Veyavo includes names of Hashem, and people did not want to mention the names if there is no requirement to do so. However, in truth, there is no prohibition in saying Ya’aleh Veyavo with Hashem’s name, as throughout the day we say prayers and supplications that include the name of Hashem. The above prohibition only applies when mentioning the name of Hashem in vain, or in an unnecessary blessing. [Admur ibid; See M”A 188:11]

[6] P”M in Noam Megadim 13:4; Ben Ish Chaiy Chukas 19; Kaf Hachaim 189:11; So is printed in the new Siddur Tehillas Hashem of 5738, although it is not brought in the Siddur of Admur nor in Admur 189:7

[7] Kaf Hachaim 208:66 in name of Yifei Laleiv 208:6; Or Zarua 181; Aguda Brachos 6:16;  So is printed in the new Siddur Tehillas Hashem of 5738

[8] So is printed in the new Siddur