The greatest historical example of papal fallibility is seen in the letter of Pope Honorius to Sergius of Constantinople. In it, he affirms monotheletism or the belief that Christ possesses only one will as opposed to dyotheletism or the orthodox belief that Jesus possesses both a human will and a divine will because he is fully God and fully man. Honorius was anathematized as a heretic by the Sixth Ecumenical Council in 681 for saying in his letter that, “We confess one will of our Lord Jesus Christ.”
What many people do not know is that Honorius’ original letter has been preserved for us today. I photocopied the Greek text while at the library of the Southern Baptist Theological Seminary. It is taken from the second volume of the Enchiridion Fontium Historiae Ecclesiae Antiquae. I have uploaded it here for those who want to see for themselves that a pope taught as doctrine that which was later condemned as heresy. The key phrase is underlined on the second page.