Comment 2 for bug 1365886

The above suggests that the bug might be triggered by r2041, though the results weren't always the same (not sure if errors in testing or bug is a race).

r2041 is to fix bug 1364725 in which there appeared to be a race in calling org.freedesktop.login1.Manager.GetSessionByPID() that would sometimes cause LightDM to not know the session ID for the session and thus never activate it. The solution was to not make this D-Bus call as we already had the session ID in the XDG_SESSION_ID environment variable (this is the technique GDM uses).