What are the connotations of "Lo Sosifu ... , ve'Lo Sigre'u mimenu"?
Rashi and Ramban #1 (citing the Sifri): 'For example, not to write five Parshiyos in one's Tefilin, not to take five species with one's Lulav or to tie five Tzitziyos to one's garment'. 1 And the same applies to "ve'Lo Sigre'u".
Ramban #2: It also includes things like sitting in the Succah on Shemini Atzeres in order to fulfill a Mitzvah, 2 or keeping Succos for only six days.
Ramban #3: Someone even transgresses if he concocts a Yom-Tov not written in the Torah. 3
Ramban: Or even to add some words to Birchas Kohanim - such "Hashem Elokei Avoseichem Yosef aleichem Elef Pe'amim".
Ramban: See Rosh Hashanah, 28b.
Ramban: Like Yerav'am ben N'vat - See Melachim 1, 12:33. See Megilah, 14a and Yerushalmi, Megilah, 1:7. See Ramban, DH 'u'Lefi Da'ati' who concludes that the prohibition against a Navi adding Mitzvos we learn from "Eileh ha'Mitzvos" (Vayikra, 27:34), and that the Takanos that the Chachamim instituted - such as Sheniyos la'Arayos - are a Mitzvah min ha'Torah (which we learn from the Pasuk in Vayikra, 18:30, "u'Shemartem es Mishmarti".