On Rosh Hashanah, the third Brachah of the Amidah has a long addition beginning with the words L’Dor V’dor. If one omitted this addition, he may go back to recite it only if he realized before saying Hashem’s name at the end of the third Brachah. Otherwise, he continues the Amidah without going back, as long as he is sure that he concluded the Brachah with the words Hamelech Hakadosh.
If one recited the unique Rosh Hashanah Nusach of the third Brachah, but is in doubt whether he concluded the Brachah with the words Hamelech Hakadosh, he may assume that he did so correctly. One who did not say Hamelech Hakadosh: If he realized before he began the next Brachah and within the time frame it takes to say three words, he corrects his mistake. Otherwise, he must begin the Amidah again. [The same applies if this occurs in Chazarat Hashatz, in which case Kedushah is recited again.]
If one mistakenly omitted any of the other four additions (Zachreinu, Mi Chamocha, u’Chtov, u’Vsefer Chaim), he may recite it at the place he remembers if he has not yet said Hashem’s name at the end of that Brachah. Otherwise, he continues the Amidah and does not go back.
During the Aseret Yemei Teshuva, most have the custom to say Oseh HaShalom at the end of the Amida and Kaddish, instead of Oseh Shalom. During Chazarat Hashatz, the congregation recites the selections of U’Chtov and u’Vsefer Chaim aloud before the Chazzan. Most also say Zachreinu Lechaim and Mi Chamocha aloud too. Most have the custom to say Le’Eilah U’Leilah in Kaddish the whole Aseret Yemei Teshuvah, the Chabad custom is to say Le’Eilah U’Leilah only in Neilah on Yom Kippur.
