Eris's ritual accidentally grants invulnerability to the feral god Meatus, making him indestructible.
Donut catches it before anyone else: Eris messed up the spell, and the feral god Meatus is invulnerable too — she has accidentally made him indestructible. It is the clearest evidence for Mordecai's read that Eris is panic casting the Ceremony of Chaos without understanding what she is doing.
Rosetta draws out the consequence that terrifies everyone: if Eris notices the error, her likely fix is to turn off just the Invulnerability Buff, which would strip the protection from eighty thousand people still locked in the Witness State and kill most of them by crushing or suffocation. The mistake is what makes The Pile a countdown rather than a stalemate.
The botched detail that turns the ritual from a nuisance into a mass-casualty threat.