When working with content catalogs, if your visitors use an invalid URL, instead of seeing the Page Not Found page they will see an empty page.
When working with cross-site publishing and content catalogs SharePoint 2013 uses search to retrieve the requested content. Using the information from the URL SharePoint 2013 attempts to find the requested catalog item and when found shows it on the screen as specified in the Catalog Item Page. Unfortunately, if no such catalog item is found, instead of returning the 404 Page Not Found response and showing the Page Not Found page, SharePoint 2013 shows an empty Catalog Item Page. An easy way to fix this undesired behavior is to extend the standard Catalog Item Reuse Web Part and after it executed its search query, verify that at least one search result has been found. If not the Web Part would return the 404 Page Not Found response and would render the Page Not Found page instead.