Horayos 2:1

If the anointed Kohein Gadol made an erroneous halachic decision for himself unwittingly, and he acted upon it unwittingly, he must offer a bull. If he ruled unwittingly but acted upon it deliberately, or vice versa, he is exempt. This is because the decision of the Kohein Gadol for himself is the same as a ruling of the court for the community.

Horayos 2:2

If the Kohein Gadol decided for himself and acted by himself, he makes atonement for himself. If he decided with the people and acted with the people, he makes atonement with the people. Just as the court is only liable if they issue a ruling invalidating part of a corpus of law while also validating part, so too the Kohein Gadol is only liable in such cases. Even in cases of idolatry, they aren’t liable until they issue a ruling invalidating part of a corpus of law while also validating part.