exists() should be isKnown()
When you do a fulltext search for something which is an exact name of a special page (see the linked URL), the search results page begins with a message like
Create the page "[[Special:AllPages]]" on this wiki!
which is obviously a nonsense.
The problem is that SpecialSearch::showCreateLink checks for Title::exists(), while it should use Title::isKnown(). (Or, even better, add a bit more logic into that function, see bug #19600.)