Berakhot 34A

חברותא כלפי שמיא מי איכא?! אי לא כוון דעתיה מעיקרא ⁠— מחינן ליה במרזפתא דנפחא עד דמכוין דעתיה.

Can one have that degree of familiarity with Heaven, to the extent that he can take his words lightly and say them however he likes? If he did not focus his attention initially, we beat him with a blacksmith’s hammer until he focuses his attention, as conduct of that sort is unacceptable.

מתני׳ (האומר: ״יברכוך טובים״⁠ — הרי זה דרכי מינות). העובר לפני התיבה וטעה ⁠— יעבור אחר תחתיו, ולא יהא סרבן באותה שעה. מהיכן הוא מתחיל? מתחלת הברכה שטעה זה.

Mishnah: This Mishnah and the next one deal with the communal prayer leader. (If one says: “May the good bless You,” this is a path of heresy.) One who is passing before the ark, as prayer leader, and erred, another should immediately pass in his place, and at that moment, this replacement should not refuse in the interest of courtesy. The Amida prayer was interrupted and he should replace him as quickly as possible. From where does the replacement commence? From the beginning of the blessing in which the former had erred.

העובר לפני התיבה לא יענה ״אמן״ אחר הכהנים, מפני הטרוף. ואם אין שם כהן אלא הוא ⁠— לא ישא את כפיו. ואם הבטחתו שהוא נושא את כפיו, וחוזר לתפלתו ⁠— רשאי.

In order to prevent the prayer leader from erring in his prayer, it was said that one who passes before the ark should not respond amen after the blessing of the priests, because of potential confusion. Since the Mishnah is describing a situation in which he was praying without a prayer book, responding amen would interrupt the order of the prayer and potentially lead him to begin a different blessing. For this reason, even if there is no priest other than the communal prayer leader, he does not lift his hands to bless the people, lest he become confused. And, however, if he is certain that he can lift his hands and resume his prayer without becoming confused, he is permitted to recite the blessing.