Have you ever heard of Pope Honorius? Apparently the church of his era had never heard of this dogma of ex cathedra infallibility.
Pope Honorius was condemned as a heretic by the 6th Ecumenical Council for what he taught in a letter, which he wrote as the bishop of Rome, to Sergius. The Council stated explicitly that Honorius and the others 1) taught the heresy, 2) in words "hurtful to the soul", and that 3) "the former Pope of Old Rome, who with the help of the old serpent had scattered deadly error."
Pope Leo II confirmed the decrees of that Council and also explicily stated that he too anathematized Honorius.
The anathema of Honorious also appears in the canons of the Council of Trullo.
It is repeated in the decree of faith of The Seventh Council.
For three hundred years this condemnation was repeated by all popes at their installation: “smites with eternal anathema the originators of the new heresy, Sergius, etc., together with Honorius, because he assisted the base assertion of the heretics.”
Honorius made a theological mistake because he was unfamiliar with the issues involved. His letters that disseminated the error, "in novel terms, amongst the orthodox people, an heresy ...” were around to mislead people for forty years before they were burned at the Council.
See for example, The Heresy of Honorius
Cordially,
A pronouncement cannot be ex cathedra if it conflicts with previously established infallible teaching. An infallible teaching, by its nature, can never be overturned.