When using Extension:DynamicPageList (third-party), this wikitext crashes if you use "mode=category":
<dpl> category = Anything mode = category </dpl>
because DynamicPageList/DPL.php line 1484 contains this line:
require_once( 'CategoryPage.php' );
This file no longer exists in MediaWiki 1.26, I guess. Delete this line, and the problem goes away.