Catholics, if the pope is infallible in matters of faith and practice?

Why did pope Honorius actively promote the Monothelite heresy? Why was Honorius posthumously condemned as a heretic by the 3rd council of Constantinople, the 2nd Council of Nicea, and again at the 4th council of Constantinople? How can a man guided by the Holy Spirit so as to be infallible when teaching faith and practice also be a heretic? It seems that the Holy Spirit should be able to do a little better than that. Or is it that the facts of history conflict with this particular dogma of the Roman Church?