Zevahim 23: The Kohen's Frontpiece (Tzitz)

The Torah teaches that Aaron will bear the sin committed with the sacred items - but what is that sin?? We know about the concern of pigul, so what is added here? Might it be an issue of purity and impurity? Also, another disqualification from the mishnah - namely, a kohen who does his part of the service while sitting instead of standing. Plus, the principle of no two verses deriving the same law, specifically common law, as derived for other cases.