papal infallibility means that the pope makes, or should make, no mistakes in matters concerning the doctrine of the Catholic Church. The Council even went so far as to state that should anyone, which God forbid, have the temerity to reject this definition [of papal infallibilityMP] of ours: let him be anathema [condemnedMP] (First Dogmatic..., 1870, 4.9).
For more than a century, this dogma has plagued many Catholics who have worked feverishly to try to harmonize the nature of the infallible dogma with the declarations, teachings, and revelations of the popes who lived before and after its establishment. After all, it is claimed that the Catholic Church does not create new dogmas; rather, it proposes eternal truths already contained in the deposit of faith (see Roman Catholic..., 1892, 8:772; Dixon, 1852, p. 197). Therefore, it could be said that, before the 19th century, every pope has been subject to infallibility without knowing it.
Italicized text from here, with plenty of examples:
http://www.apologeticspress.org/APContent.aspx?category=11&article=2594
unfortunately I think Protestants, especially those who don’t care for Roman Catholicism, have latched on to and give much more credence and faith to ‘papal infallibility’ than actual Catholics do. It is a convenient whip to use to attack, especially on an easily misunderstood doctrine.