This sugya comes up as the Torah Temima's citation #12 on Parshas Vayikra 4:3.

How do we get to the situation that both the Kohein Moshiach and Beis Din ruled on the same matter? The Kohein cannot rule if Beis Din ruled (we see something like this on 12A - cited earlier - that when the Kohein relies on Beis Din for a matter that he thinks is in error he is exempt), and similarly Beis Din is not likely to consult the Kohein (since he is under their authority) and consider it a Kohein's ruling (consultant is their advisor, not their authority), so perhaps it must be that the Kohein ruled and the Kohein asked the Beis Din to validate his ruling. His ruling was an error, but they erroneously validated it, so they both ruled.

Thus we have the chiddish that even though he ruled initially, once Beis Din made it their ruling, he's effectively just part of the community relying on their ruling?

Could it possibly be that Kohein Moshiach acts as Beis Din's deputy, and therefore his ruling is their ruling, even though he rules and acts, we see his ruling as an agent of Beis Din, and his action as the community relying upon Beis Din?

Note further, I cannot also help but note that I learned this sugya before Shabbos of Parshas Korach. Could this perhaps give us a slight insight into Korach's reasoning for involving 250 followers? Mrs. On pointed out that On had nothing to gain by taking part in the dispute. Perhaps if Korach ruled and he had followers and Korach himself acted with the community of followers, he thought of himself as just as exempt as the Kohein Moshiach would be?

Perhaps this explains why Moshe reacted to the challenge to Aharon, as Kohein Gadol, even though Rashi tells us that Korach's sights were set on the position of leader of Levi? That Korach wanted to be Kohein Moshiach?

Chaim Chesler.