When Rosh Chodesh falls on Erev Shabbos: Even when Rosh Chodesh falls on Erev Shabbos some communities are accustomed not to cut their hair in honor of Shabbos due to the above warning of danger of Rav Yehuda Hachassid. [See Q&A regarding cutting nails in such an instance!].
May one cut his nail when Rosh Chodesh falls on Erev Shabbos?
Some Poskim rule it is permitted to cut the nails on Erev Shabbos Rosh Chodesh and so is implied from Admur. Other Poskim however rule one may not cut the nails. [Practically, one who cuts his nails on every Erev Shabbos may be lenient to do so also on Erev Shabbos Rosh Chodesh. Likewise, one with very long nails may be lenient to cut them.]
May a woman shave her legs and the like on Rosh Chodesh?
If one began cutting his nails or hair and then remembered that it is Rosh Chodesh may he finish cutting them?
If one began cutting his nails [or hair] on Rosh Chodesh and then realized his mistake, he may continue to cut the remainder, and Hashem will protect him of his mistake. This applies even if he began cutting only a small part of the nail.
Is a boy who was born on Rosh Chodesh to have his Upsherinish haircut take place on his third birthday on Rosh Chodesh?
The hair is not to be cut on Rosh Chodesh but rather immediately after Rosh Chodesh.
 Tzavas Rav Yehudah Hachasid 48 [hair] 49 [nails] brought in: Olas Shabbos 260/1; M”A 260/1 and Admur 260/1 regarding [only] hair; Beir Heiytiv 260/2 [regarding hair and nails]; Kneses Hagedola 260 regarding hair.
 Kneses Hagedola ibid writes “forbidden”, however the wording in Rav Yehuda Hachassid ibid and Beir Heiytiv is “One may not”; In Admur ibid the wording is simply that “Some places are accustomed” however this is referring to Erev Shabbos and not to every Rosh Chodesh.
 See Q&A regarding women shaving.
 Rav Yehuda Hachassid 49; Beir Heiytiv ibid; Kneses Hagedola ibid; omitted from Admur ibid and M”A ibid; The Ketzos Hashulchan 73 footnote 4 questions as to why no mention of this is made by the M”A or Admur in 260/1. See however Shaar Hakolel in Nesiv Hachaim 48/3; Shivim Temarim 57 who explain Admur omitted it because on Erev Shabbos it is Mitzvah to cut the nails even when it coincides with Rosh Chodesh. However on a non-Erev Shabbos Rosh Chodesh he would agree that one may not cut the nails. See next!
 The reason: As Rav Yehudah Hachasid warned that doing so involves danger. [Admur ibid; Beir Heiytiv ibid; See Likkutei Sichos 13/264]
The night of Rosh Chodesh: See Ashel Avraham Butchach Tinyana 417 that the night of Rosh Chodesh is more lenient in regards of cutting nails.
 Admur 260/1; Kneses Hagedola 260; M”A 260; Beir Heiytiv 260/2
Other Opinions: Some Poskim rule it is permitted to cut one’s hair in honor of Shabbos even if Erev Shabbos is Rosh Chodesh, as one who fulfills a Mitzvah will know no harm. [Tehila Ledavid 260/1]
 Tehila Ledavid 260/1; Shaar Hakolel in Nesiv Hachaim 48/3; Shivim Temarim 57 as is the simple implication of the Kneses Hagedola ibid; Beir Heiytiv ibid; M”A and Admur ibid which omit the custom to avoid cutting nails when mentioning the custom to avoid cutting hair on Erev Shabbos Rosh Chodesh; opinions brought in footnotes on the Tzavah ibid; See Minchas Shabbos 72/66; Alef Hamagen 12; Maharam Brisk 2/99; Meishiv Halacha 2/4; Kitzur Hilchos Shabbos-Supplements.
The reason: If Rosh Chodesh falls on Erev Shabbos, the reason some have the custom not to cut their hair, despite the fact that this is usually a Mitzvah to do on Erev Shabbos, is because one is not obligated to cut his hair every Erev Shabbos. However being it is a Mitzvah to cut the nails on every Erev Shabbos, one is allowed to cut his nails even on Erev Shabbos Rosh Chodesh as one who fulfills a Mitzvah will know no harm. It thus seems that no such custom exists against cutting the nails on Erev Shabbos Rosh Chodesh and they should therefore be cut in honor of Shabbos. [Shivim Temarim ibid]
Other opinions: The Ketzos Hashulchan 72 footnote 4 leaves the opinion of Admur on this matter in question.
 M”B 260/7; Aruch Hashulchan 260/6; Kitzur SHU”A 72/14; Kaf Hachaim 260/12 allows only cutting the nails that stretch past the skin.
 Shivim Temarim ibid
 Kaf Hachaim ibid
 Tehila Ledavid 260/1
 The reason: As one who does a Mitzvah will see no harm.
 Tzavas Rav Yehudah Hachasid ibid: “A person is not to shave his head or beard on Rosh Chodesh”
 Ashel Avraham Butchach 417 Tinyana; brought in Ketzos Hashulchan 73 footnote 4
 Igros Kodesh 18/191; 22/372 [printed in Shulchan Menachem 2/202]; Kapos Temarim 14 [on the Sefer Shivim Temarim]