The "Fork me on Github" ribbon creates a rectangular anchor tag space that overlays over elements on the page. When this occurs, it means that trying to navigate to things under the rectangular area results in being directed to the Jenkins Github pages.
This problem is most noticeable in Safari on OSX on plugins.jenkins.io when viewing the plugins list as the ribbon overlays the pagination elements. It issue is also noticeable on, say, the Jenkins Blog page as it overlaps the top event box. Note that specifically for the plugins.jenkins.io case this is not an issue with Chrome or Firefox but only because the ribbon is somehow not displayed for those two browsers.
Not sure of a real solution other than making the ribbon only the size of the navigation bar, making the navigation bar the size of the ribbon, or ensuring that there is only clear space in the area taken by the ribbon.