Why was Pope Honorius I criticized in discussions of papal infallibility?

Pope Honorius I is frequently cited by critics of papal infallibility because he was posthumously condemned as a heretic by the Third Council of Constantinople (680–681).12

During his pontificate, Honorius became involved in the monothelite controversy, which concerned whether Christ possessed one or two wills. When asked to intervene, Honorius supported the doctrine of "one will" and forbade further discussion of the matter. Following his death, the Third Council of Constantinople decreed that Christ had two wills and condemned Honorius’s teachings as pro-monothelitic, a judgment confirmed by Pope Leo II.12

Opponents of papal infallibility used this historical case during the First Vatican Council to argue that popes could err in matters of faith. Conversely, defenders of the doctrine have argued that Honorius’s statements were not official ex cathedra pronouncements, suggesting he may have simply misunderstood the theological issues or used imprecise language rather than intending to teach heresy.12