What is the law if one forgot to say the verse of Adono-y Sefasaiy Tiftach and already began Shemoneh Esrei?
Some Poskim rule he nevertheless fulfills Shemoneh Esrei. Other Poskim, however, leave this matter in question. [Practically, one can Daven again as a Tefilas Nidava to escape the dispute, or pay attention to the Chazan’s repetition, and recite the verse to himself prior to the Chazan beginning.]
Must one repeat the verse of Adono-y Sefasaiy Tiftach if he made a mistake in Shemoneh Esrei and is repeating from the beginning?
If a mistake was made in Shemoneh Esrei which requires one to repeat it from the beginning, then if he did not yet complete Shemoneh Esrei, and will immediately restart from the beginning, there is no need to repeat the verse of “Ado-naiy Sefasaiy Tiftach”. If, however, he already finished Shemoneh Esrei, then he must wait for the amount of time it takes to walk four Amos and then repeat the verse of Adniy Sefasaiy Tiftach prior to restarting.
 Biur Halacha 111:2 “Chozer Viomer”; Toras Yekusiel 53; Piskeiy Teshuvos 111:2
 Igros Moshe 5:24
 Shaareiy Teshuva 114:2 in name of Ritva Taanis; M”B 114:21; Ketzos Hashulchan 21:3
Other opinions: Some Poskim rule that one is to repeat the verse of Adniy Sefasaiy Tiftach. [Aruch Hashulchan 114:8]
 Pirush Rebbe Yonason Milunil on Rif Brachos 20; Ketzos Hashulchan 21:3 and footnote 6; 78 footnote 19; Afikei Maginim 114:17; Igros Moshe 5:24; Piskeiy Teshuvos 114:10
Other opinions: Some Poskim rule that one is not required to repeat the verse of Adniy Sefasaiy Tiftach even if he finished Shemoneh Esrei. [Shalmei Tzibur p. 222; Shaareiy Teshuvah 422:4; Kaf Hachaim 422:15]