Manual:$wgAllowDisplayTitle

Details
When this option is enabled, users will be able to use the DISPLAYTITLE magic word to override the title of a page, provided the selected title normalises to the same title.

For example, on a page titled IPod, one can use   to force the title to appear as "iPod" when viewed. However,   will not work, because "Foo" is not the same title as "IPod".

Since MediaWiki 1.14, it is possible to disable this restriction and thus allow any display title by setting $wgRestrictDisplayTitle = false; This is not recommended however, as it breaks the wiki convention that a page's title is it's name, and thus can be used for linking to it.

$wgAllowDisplayTitle was disabled by default in MediaWiki until 1.10.0, as previous implementations were buggy, incompatible with caching, and didn't perform title normalisation checks, which are essential to allow correct linking to articles.