A fellow is praying the evening Maariv the night after Rosh Chodesh. In error he said Yaaleh V'Yavo. He realizes when he finished Shemona Esrei. Does he have to say over Shemona Esrei?
3 Answers
According to the Chayei Adam (25:11,13; 28:13, and quoted in Mishnah Berurah 108:38), one does not repeat Shemona Esrei in such a scenario.
According to Shulchan Aruch Harav (OC 108:16), since there's a dispute whether he needs to go back he should repeat Shemona Esrei as a voluntary prayer, but does not need to add anything to the blessings (which would normally be the case by a voluntary prayer, see Shulchan Aruch OC 107).
Mishnah Berurah (108:38) rules out that you can’t repeat. (Though he does present dissenting opinions at first.)
בין לענין שיחה בשוגג בתפלה ובין לענין שהזכיר דבר שלא בזמנו בדיעבד אם לא חזר לתחלת הברכה כיון שסיים הברכה אף שהוסיף בה דבר שלא כענינו אינו רשאי לחזור וכ"ש אם כבר השלים תפלתו
I remember i had this question in yeshiva (sorry no written sources) A rebbi of mine called a prominent posek who said it was fine and there is no need to repeat shmona esre.