If the person is other than the only Begotten of the Father, then he or she is fallible, by definition of being human.
Matthew 16:19:
And I will give you the keys of the kingdom of heaven, and whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed[d] in heaven.”
Vatican 1:
Therefore,
faithfully adhering to the tradition received from the beginning of the christian faith,
to the glory of God our saviour,
for the exaltation of the catholic religion and
for the salvation of the christian people,
with the approval of the sacred council,
we teach and define as a divinely revealed dogma that
when the Roman pontiff speaks EX CATHEDRA,
that is, when,
in the exercise of his office as shepherd and teacher of all Christians,
in virtue of his supreme apostolic authority,
he defines a doctrine concerning faith or morals to be held by the whole church,
he possesses,
by the divine assistance promised to him in blessed Peter,
that infallibility which the divine Redeemer willed his church to enjoy in defining doctrine concerning faith or morals.
Therefore, such definitions of the Roman pontiff are of themselves, and not by the consent of the church, irreformable.