Unified login feature randomly appears to not work. This bug appears especially on wikidata.org, wikimedia.org and mediawiki.org (other Wikimedia projects respond with login flyout more frequently).
In a nutshell: I am visiting one of Wikimedia sites (e. g. Wikidata) while signed in Wikipedia, but Wikidata doesn’t sign me in. This can lead to confusion since Wikidata interface is fully on AJAX (so user could not notice relatively small warning about being anonymous).
Issue appears in Firefox 47 (Windows 10). None of my browser addons interfere with cookies at Wikimedia sites.
See Also: T139298: Central Login to Commons buggy