The following query:
http://commons.wikimedia.org/w/api.php?action=query&list=allcategories&acprefix=Rept
yields a list that contains "Reptil", "Reptile", and "Reptiles".
I've developed a clientside solution for testing for and traversing the "soft" category redirects that may show up in the category listing (e.g. http://commons.wikimedia.org/wiki/Category:Reptiles).
However, the pages "Reptil" and "Reptile" do not even exist. It is problematic when these entries are presented to the user in the form of "autocomplete" search suggestions; if the user selects one of these entries, I could do a test for page existance, but a failed test is disruptive as it forces the user to make another choice, leaving them to wonder why it was offered in the first place.
My current project is making a free game for Android that has users categorize the images here:
http://commons.wikimedia.org/wiki/Category:Media_needing_categories
Karl