When accessing any Phabricator path, including /, users that are not logged in see this exception:

PhabricatorPolicyException
[Access Denied: Restricted Application] (Can View) You do not have permission to view this object. // Logged in users can take this action.

The login form is not displayed. Users that have an active session can access Phabricator normally.

Version Information
phabricator 9d3f09ab478e44d30b6a58688c470239b44efb02 (Tue, Jan 17)
arcanist ade25facfdf22aed1c1e20fed3e58e60c0be3c2b (Thu, Jan 5)
phutil 9d85dfab0f532d50c2343719e92d574a4827341b (Thu, Jan 12)

This was also posted at Q554: EXCEPTION: PhabricatorPolicyException [Access Denied: Restricted Application] (Can View) .