I also having this bug. System is not shutting down properly. 

rsyslog file:

May 23 22:43:22 jsm NetworkManager[666]: <info> (ttyACM0): device state change: activated -> disconnected (reason 'user-requested') [100 30 39]
May 23 22:43:22 jsm NetworkManager[666]: <info> (ttyACM0): deactivating device (reason 'user-requested') [39]
May 23 22:43:22 jsm pppd[2335]: Terminating on signal 15
May 23 22:43:22 jsm pppd[2335]: Connect time 33.5 minutes.
May 23 22:43:22 jsm pppd[2335]: Sent 816683 bytes, received 23488634 bytes.
May 23 22:43:22 jsm NetworkManager[666]: nm_system_iface_flush_routes: assertion `ifindex > 0' failed
May 23 22:43:22 jsm NetworkManager[666]: nm_system_iface_flush_addresses: assertion `ifindex > 0' failed
May 23 22:43:22 jsm pppd[2335]: Connection terminated.
May 23 22:43:22 jsm avahi-daemon[663]: Withdrawing workstation service for ppp0.
May 23 22:43:22 jsm dnsmasq[2388]: exiting on receipt of SIGTERM
May 23 22:43:22 jsm NetworkManager[666]: <info> DNS: starting dnsmasq...
May 23 22:43:22 jsm NetworkManager[666]: <info> (ttyACM0): writing resolv.conf to /sbin/resolvconf
May 23 22:43:23 jsm dnsmasq[3230]: started, version 2.59 cache disabled
May 23 22:43:23 jsm dnsmasq[3230]: compile time options: IPv6 GNU-getopt DBus i18n DHCP TFTP conntrack IDN
May 23 22:43:23 jsm dnsmasq[3230]: warning: no upstream servers configured
May 23 22:43:23 jsm modem-manager[642]: <info>  Modem /org/freedesktop/ModemManager/Modems/0: state changed (connected -> disconnecting)
May 23 22:43:23 jsm dbus[508]: [system] Activating service name='org.freedesktop.nm_dispatcher' (using servicehelper)
May 23 22:43:23 jsm NetworkManager[666]:    SCPlugin-Ifupdown: devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
May 23 22:43:23 jsm dbus[508]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
May 23 22:43:24 jsm modem-manager[642]: <info>  Modem /org/freedesktop/ModemManager/Modems/0: state changed (disconnecting -> registered)
May 23 22:43:27 jsm modem-manager[642]: <info>  (ttyACM0) closing serial port...
May 23 22:43:27 jsm modem-manager[642]: <info>  (ttyACM0) serial port closed
May 23 22:43:27 jsm kernel: [ 2975.160088] usb 3-1: USB disconnect, device number 2
May 23 22:43:27 jsm modem-manager[642]: <info>  (tty/ttyACM0): released by modem /sys/devices/pci0000:00/0000:00:1a.0/usb3/3-1
May 23 22:43:27 jsm modem-manager[642]: <info>  Modem /org/freedesktop/ModemManager/Modems/0: state changed (registered -> disabled)
May 23 22:43:27 jsm NetworkManager[666]: <info> (ttyACM0): now unmanaged
May 23 22:43:27 jsm NetworkManager[666]: <info> (ttyACM0): device state change: disconnected -> unmanaged (reason 'removed') [30 10 36]
May 23 22:43:27 jsm NetworkManager[666]: <info> (ttyACM0): cleaning up...
May 23 22:43:27 jsm NetworkManager[666]: <info> (ttyACM0): taking down device.
May 23 22:43:27 jsm NetworkManager[666]: <info> Unmanaged Device found; state CONNECTED forced. (see http://bugs.launchpad.net/bugs/191889)
May 23 22:43:27 jsm NetworkManager[666]: <info> Unmanaged Device found; state CONNECTED forced. (see http://bugs.launchpad.net/bugs/191889)
May 23 22:43:36 jsm kernel: Kernel logging (proc) stopped.
May 23 22:43:36 jsm rsyslogd: [origin software="rsyslogd" swVersion="5.8.6" x-pid="504" x-info="http://www.rsyslog.com"] exiting on signal 15.

And my machine configuration is
Processor - Intel® Core™2 Duo CPU E4600 @ 2.40GHz × 2 
OS type    - 64Bit
Memory   - 2GB
 with regards,
Subramanian