The same set of passages in Sefer Vayikra that teach about the unique responsibilities of the High Court when ruling on issues that may cause a large swath of the Jewish community to sin (see Vayikra 4:13-21), includes two other situations where out-of-the ordinary sin offerings will be brought by individuals who mistakenly err in their judgment and their actions. These two individuals are the High Priest (ha-kohen ha-mashiah – see Vayikra 4:1-7) and the king (nasi see Vayikra 4:22-26). The focus of the second perek (=chapter) of Masechet Horayot, which begins on today’s daf (=page) is on these two unique individuals and the circumstances under which they would bring these sacrifices.
The first Mishnah opens with the kohen gadol, teaching that he will bring his sin offering only if both the decision and the forbidden action were made in error. The Mishnah teaches specifically that if either were done purposefully – if the decision to permit a forbidden action were done intentionally, but the act was done accidentally or if the decision was made by accident but the act was done on purpose – then the sacrifice would not be brought.
Tosafot question how it is possible to have a situation where the decision to permit the act was done with malice aforethought, yet the act is considered to be accidental. Since the kohen gadol has the status to rule on questions of Jewish law for himself, if he purposely ruled incorrectly then it follows that the forbidden act would be done on purpose, as well. The Ramah suggests that we are discussing a case where at the time of the action he remembered that he had once ruled in this situation, but did not remember his reasoning. Thus, at the time of his action he believed that he was performing a permissible act based on an earlier, reliable ruling. The Tzofnat Pa’ane’ah, based on the discussion in the Talmud Yerushalmi, suggests that we are discussing a case where the kohen gadol was relying on the ruling of a previous High Priest, so it is clear that the erroneous ruling could have been made on purpose, but the forbidden act may have been done accidentally.