Last modified: 2014-06-02 22:18:09 UTC
See e.g. http://pl.wikipedia.org/w/api.php?action=query&prop=revisions&rvprop=comment|content&rvdifftotext=Foo&titles=Wikipedysta:Marcgal/brudnopis . There's no diff. Removing the rvprop parameter (or setting rvprop=comment instead) makes the diff appear.
*** This bug has been marked as a duplicate of bug 55371 ***