At the mentioned URL, http://codex.wordpress.org, when linking to another page
on the wiki, the exact same capitalization as the title of the page needs to be
used currently. URLs with differring cases work just fine when entered into the
address bar of the browser (and when the page is linked-to from external websites.
This causes lots of maintenance problems, and makes it difficult to edit/create
content, since one always has to be extremely careful about the case when
linking to a page.
Symptoms with an example:
If there exists a page titled "The Loop" on the wiki, and someone links to that
page using [[The Loop]] or [[The_Loop]], the link works fine.
However, if someone links to that page using [[the loop]], [[the_loop]], [[The
loop]], [[The_loop]], [[the Loop]] or [[the_Loop]] the link shows up in the
color red upon saving, and links to the "edit page"
(..index.php?title=the_loop&action=edit for example).
This problem exists only with incorrectly capitalized links entered by editors.
On Special: pages, and "what links here" pages, for example, incorrectly
capitalized links work just fine.
The page http://codex.wordpress.org/User:MichaelH/Wiki_Tips_and_Tricks contains
a list showing intra-wiki links that work and that do not, for your reference.