Manual:$wgAllowDisplayTitle/en

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

For example, on a page titled, 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. This is not recommended however, as it breaks the wiki convention that a page's title is its name, and thus can be used for linking to it.

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