In SharePoint 2013 you can create new Search Navigation links. This controls the options you get in the search dropdown box. OOTB, the options are Everything, People, Conversations and Video.
Let say you have a Portal site collection (http://portal), and a Search Centre site collection (http://portal/sites/search). If you want to configure Search Navigation links in your Portal site, and reuse the results pages in the Search Centre, you could use the following as the URL for one of your Search Navigation links: http://portal/sites/search/Pages/results.aspx. When you try to apply the setting however, you will get an error. In the ULS you will see that the error is “Cannot open “/sites/search/Pages/results.aspx”: no such file or folder.” The same error occurs if you enter an absolute URL (http://portal/sites/search/Pages/results.aspx), or a server-relative one (/sites/search/Pages/results.aspx).
So how do you overcome this? Well, the trick is to enter a URL that SharePoint will accept, then edit the link, and change the URL to what you want.
So when creating a new link, enter an absolute URL that is EXTERNAL to SharePoint, e.g. just enter http://www.google.com. Go ahead and apply the setting (that is click OK on the Search Settings page). Because the URL is external, SharePoint will not validate the link. Next, go back into the Search Settings page and edit the link. This time change the URL of the link to what you want. Go ahead and apply the setting again. This time SharePoint will happily accept it and your Search Navigation link will work as expected.
This is clearly a MS bug. I tested it on March 2013 Public Update and it is still there.