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 was 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][1]:

>  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)][2]:

> "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)

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][3],  - 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://www.newadvent.org/cathen/05413a.htm
  [2]: http://en.wikipedia.org/wiki/First_Vatican_Council
  [3]: http://www.vatican.va/holy_father/pius_x/encyclicals/documents/hf_p-x_enc_19070908_pascendi-dominici-gregis_en.html