Underscore[]
For some reason, the wikia doesn't like the underscore in Jhonny_DC's name, even though it works just fine in the display title thingy. Need a little help :P You... you killed my Daffy... (talk) 08:58, May 16, 2015 (UTC)
Because the underscore denotes a space in the URL. DISPLAYTITLE doesn't affect the URL of the page, which is why it works there.
DISPLAYTITLE doesn't work on category or template pages. I dunno about MediaWiki pages (I assume not).