Berakhot 29A

והשקיף בה שתים ושלש שעות, ולא העלוהו.

and scrutinized it, in an attempt to remember the blessing for two or 3 hours, and they did not remove him from serving as prayer leader.

אמאי לא העלוהו? והאמר רב יהודה אמר רב: טעה בכל הברכות כלן ⁠— אין מעלין אותו, בברכת המינים ⁠— מעלין אותו. חיישינן שמא מין הוא?

The Talmud asks: Why did they not remove him? Didn’t Rav Yehuda say that Rav said: One who was serving as the prayer leader before the congregation and erred in reciting any of the blessings, they do not remove him from serving as the prayer leader. However, one who erred while reciting the blessing of the heretics they remove him, as we suspect that perhaps he is a heretic and intentionally omitted the blessing to avoid cursing himself. Why, then, did they not remove Shmuel HaKatan?

שאני שמואל הקטן דאיהו תקנה.

The Talmud answers: Shmuel HaKatan is different because he instituted this blessing and there is no suspicion of him.

וניחוש דלמא הדר ביה. אמר אביי: גמירי, טבא לא הוי בישא.

The Talmud continues: Let us suspect that perhaps he reconsidered and, although he had been righteous, he had a change of heart? Abaye said: We learned through tradition that a good person does not become wicked.

ולא?! והכתיב: ״ובשוב צדיק מצדקתו ועשה עול״. ההוא רשע מעיקרו, אבל צדיק מעיקרו ⁠— לא.

The Talmud challenges this: And does he not become wicked? Isn’t it explicitly written: “And when the righteous one returns from his righteousness and does wicked like all of the abominations that the wicked one has done, will he live? All of the righteous deeds that he has done will not be remembered given the treachery that he has carried out, and in his sin that he has transgressed, for these he should die” (Ezekiel 18:24)? Abaye responds: That verse refers to a righteous individual who was initially wicked and repented, but ultimately returned to his evil ways. However, one who is initially righteous does not become wicked.