Start a new book and click "add this page to book"
Notice:
load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=0cg1aij:154 Use of "wgNamespaceNumber" is deprecated. Use mw.config instead. get @ load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=0cg1aij:154 onclick @ W._J._Cash:53 load.php?debug=false&lang=en&modules=jquery%2Cmediawiki|mediawiki.legacy.wikibits&only=scripts&skin=vector&version=0cg1aij:154 Use of "wgTitle" is deprecated. Use mw.config instead.
It appears this is using globals. This will break soon (@Krinkle probably can inform us on when) and if that happens this feature will be broken.