If your Rabbi tells you something about Torah and you know that he is incorrect, and this has happened in the past and the Rabbi admitted his mistake, are you then allowed to correct the Rabbi if a third person is present?
One should correct him in an indirect way, reminding him of his earlier correct teaching or asking a question that leads in the right direction.
This is based on the Shulchan Aruch in YD 242:22 (my translation)
ראה רבו עובר על דברי תורה אומר לו למדתני רבינו כך וכך
The one sees his Rav make a (Torah) mistake tells him "Rabbi, you taught us so and so"
It is also in line with what the Rambam writes (MT Mamrim 6:11)
If he sees his father violate Torah law, he should not tell him: 'Father, you transgressed Torah law.' Instead, he should tell him: 'Father, is not such-and-such written in the Torah?', as if he is asking him, rather than warning him.