Google is now showing what they call "sub-sitelinks" for some searchers in the search results. Sitelinks are the links you find under a search result that show links to popular pages within the site. This often comes up for navigational searches, for example a search on [search engine roundtable] and tons of other queries.
Google is now showing some searchers sub-sitelinks, so sitelinks for sitelinks. Here is an example of it in action provided by Google Operating System blog. As you can see by the animated GIF I made, the sitelinks show with arrows down, when clicked, they show secondary sitelinks.
Here is a larger image provided by @6cSocial:
"Sub-sitelinks" in expanded sitelinks. [launch codename "thanksgiving"] This improvement digs deeper into megasitelinks by showing sub-sitelinks instead of the normal snippet.
I personally do not see sub-sitelinks and the people who have, only have seen it on the mobile search results.
Update: This is now official, Google's search blog covered it.