If you upgrade from an older version of Joomla! / Gary's Cookbook you might want to use old extensions in the new enviroment. Then you can activate the so called Joomla! legacy mode. BUT --> This is not THE solution. It might work or might not. You have to thoroughly test every
extension if it still works. Extensions manupulating the Html-code are probably NOT working!
Please do not look for problemreasons in native solutions like the Cookbook in the first place!