Does not look like this is fixed. I just upgraded from 14.04 to 16.04.1 and my syslog gets spammed with 

Nov 21 14:36:09 bnasws02 gnome-session[1536]: Error executing command as another user: Not authorized
Nov 21 14:36:09 bnasws02 gnome-session[1536]: This incident has been reported.
Nov 21 14:36:10 bnasws02 gnome-session[1536]: Error executing command as another user: Not authorized
Nov 21 14:36:10 bnasws02 gnome-session[1536]: This incident has been reported.
Nov 21 14:36:22 bnasws02 gnome-session[1536]: Error executing command as another user: Not authorized
Nov 21 14:36:22 bnasws02 gnome-session[1536]: This incident has been reported.
Nov 21 14:36:23 bnasws02 gnome-session[1536]: Error executing command as another user: Not authorized
Nov 21 14:36:23 bnasws02 gnome-session[1536]: This incident has been reported.
Nov 21 14:36:31 bnasws02 gnome-session[1536]: Error executing command as another user: Not authorized
Nov 21 14:36:31 bnasws02 gnome-session[1536]: This incident has been reported.

my auth.log with:

Nov 21 14:36:43 bnasws02 pkexec[24666]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 927]
Nov 21 14:36:43 bnasws02 pkexec: pam_unix(polkit-1:session): session opened for user root by (uid=1000)
Nov 21 14:36:43 bnasws02 pkexec: pam_systemd(polkit-1:session): Cannot create session: Already running in a session
Nov 21 14:36:43 bnasws02 pkexec: pam_ck_connector(polkit-1:session): cannot determine display-device
Nov 21 14:36:43 bnasws02 pkexec[24667]: sebastian: Executing command [USER=root] [TTY=unknown] [CWD=/home/sebastian] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 927]
Nov 21 14:36:47 bnasws02 pkexec[24675]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 927]
Nov 21 14:36:47 bnasws02 pkexec: pam_unix(polkit-1:session): session opened for user root by (uid=1000)
Nov 21 14:36:47 bnasws02 pkexec: pam_systemd(polkit-1:session): Cannot create session: Already running in a session
Nov 21 14:36:47 bnasws02 pkexec: pam_ck_connector(polkit-1:session): cannot determine display-device
Nov 21 14:36:47 bnasws02 pkexec[24676]: sebastian: Executing command [USER=root] [TTY=unknown] [CWD=/home/sebastian] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 927]
Nov 21 14:36:50 bnasws02 pkexec[24685]: gdm: Error executing command as another user: Not authorized [USER=root] [TTY=unknown] [CWD=/var/lib/gdm3] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 927]
Nov 21 14:36:50 bnasws02 pkexec: pam_unix(polkit-1:session): session opened for user root by (uid=1000)
Nov 21 14:36:50 bnasws02 pkexec: pam_systemd(polkit-1:session): Cannot create session: Already running in a session
Nov 21 14:36:50 bnasws02 pkexec: pam_ck_connector(polkit-1:session): cannot determine display-device
Nov 21 14:36:50 bnasws02 pkexec[24686]: sebastian: Executing command [USER=root] [TTY=unknown] [CWD=/home/sebastian] [COMMAND=/usr/lib/gnome-settings-daemon/gsd-backlight-helper --set-brightness 927]