Summary
- Some statement X was made, intended to be infallible
- Later, 1) evidence has shown X to be false or 2) X was retracted/reformulated/revised by a subsequent statement, illustrating that the previous statement was obviously not infallible
It is not possible to find a simple contraction like this, because the statement is mostly not marked as "infallible" when made, its infallibility is only derived from context. There are only two statements which are not disputed regarding their "ex cathedra" status, and those are two recent dogmas of the Immaculate Conception (1854) and the Assumption (1950). See [Instances of papalinfallibility in Wikipedia][1].
With any other statement which you may think was made ex-cathedra in its time and can be shown as not held any more, or not true, the proponents of the papal infallibility faith will claim it was not made "ex cathedra" at all.
Discussion
A trouble is there is not a clear consensus between catholics regarding how to recognize when a statement is made Ex cathedra, therefore I may easily happen a statement which might had been considered as ex cathedra at the time it was issued is classified as an ordinary statement later. See e.g. [Encyclical entry in the New Advent Catholic Encyclopedia][2]:
it is generally admitted that the mere fact that the pope should have given to any of his utterances the form of an encyclical does not necessarily constitute it an ex-cathedra pronouncement and invest it with infallible authority. The degree in which the infallible magisterium of the Holy See is committed must be judged from the circumstances, and from the language used in the particular case.
The currently used definition comes from [First Vatican Council (1869-1870)][3]:
"When the Roman Pontiff speaks ex cathedra...as the pastor and teacher of all Christians [and] defines a doctrine of faith and morals that must be held by the Universal Church, he is empowered, through the divine assistance promised him in blessed Peter, with the infallibility with which the Divine Redeemer willed to endow his Church."(24)
Extensive discussion of the problem with sources (mostly literature not available online) can be found in [PROKOFIEFF'S THE CASE OF VALENTIN TOMBERG][4]:
Interpreting this statement, as well as determining which papal proclamations are to be considered "ex cathedra" or infallible and therefore irrevocable and binding, has not proven easy historically. Rudolf Steiner reflects this ambiguity in stating that the pope never says whether he is speaking "ex cathedra" or not (Rudolf Steiner, GA 198, lecture of June 3, 1920)
Note: The belief in "Papal infallibility" did not start with First Vatican, it was held before, but it was not rigorously formulated (TODO: add sources here).
I have one particular example in mind, and that is [Pascendi Dominici Gregis, 1907][5], - encyclic against modernism, which some conservative catholic groups consider as "ex cathedra", but it is not considered as such by nowadays mainstream catholic church.
Note: ... I will try to extend the answer later, and provide definitions and links, too busy now.
[1]: http://en.wikipedia.org/wiki/Papal_infallibility#Instances_of_papal_infallibility [2]: http://www.newadvent.org/cathen/05413a.htm [3]: http://en.wikipedia.org/wiki/First_Vatican_Council [4]: http://home.comcast.net/~vtsophia/casevt1.htm#N_26_ [5]: http://www.vatican.va/holy_father/pius_x/encyclicals/documents/hf_p-x_enc_19070908_pascendi-dominici-gregis_en.html