What I mean by that is that it looks a lot like old german emergency exit signs (http://www.zeichnen-lernen.net/bilder/06-zeichnen-gestalten/10-semantik-zeichenlehre/beispiel-notausgang.png, https://de.wikipedia.org/wiki/Rettungszeichen#mediaviewer/File:Rettungszeichen_Notausgang_Links_(alt).svg) and it doesn't have a tooltip (neither do the other icons in the top-right corner once you've logged in, btw). It also doesn't load if browser addons like NoScript are used. When I first wanted to report a bug to a project that uses Phabricator, it took me several minutes (!) to figure out how to login and I'm used to all kinds of bug-trackers (mantis, jira, trac, ...). I asked several other people who're used to bug tracking software as well, and all of them were surprised that the "exit" icon was the one which lets you login.
The users guide (https://secure.phabricator.com/book/phabricator/) did not help either because it seems to be written for people who want to run a Phabricator instance or already have a basic understanding of how Phabricator works (it starts with installation, configuration and then application specific user guides). Using the search on the users guide page to search for "login" or "sign in" does not produce any results that document how loggin in works.