It’s a very good question. It’s related to the question of whether when the Beis Din Hagadol issues a mistaken ruling and they must bring a ?? ???? ???, does the rest of klal yisroel still have to bring personal ?????? I believe that that is a machlokes rishonim and if I’m not mistaken R’ Chaim handels it, though I don’t have seforim in front of me. Bear in mind, however, that in your scenario it seems much more likely that the person following the ruling is closer to a classic shogeg than the case I mentioned, because our rabbanim do not have the authority to make something halacha to the extent that it would be included in ?? ????; only the Beis Din Hagadol does.
If something is truly a bad psak, you certainly may not follow it. Though if a person is ???? ?????? and he has a ???? ????? you can certainly rely on him, even if he’s made a mistake in the past. Everyone makes mistakes.