Latin: "The Emperor is not above the grammarians".
At the Council of Konstanz (1414), Holy Roman Emperor Sigismund misused the word schisma ("division, dissent"), which is ungendered (neutrum), as if it were feminine (femininum) (apparently misled by the fact that schisma ends in an "-a").
When attention was called to his error, Sigismund decided, unwisely, to invoke his imperial authority, and declared that, henceforth, schisma would be femininum.
This was too much for Archbishop Placentinus, who got creakily to his feet and declared: "Caesar non supra grammaticos." - and that was the end of that.