Lubuntu 13.04 had this problem with 3.8.0.28-generic (i386) a couple of weeks ago affecting 3 machines on my LAN.
A reboot found the MBR was corrupt on 1 of 3 machines, so maybe something else caused that.
It happened again with 3.8.0-29-generic this week.
sudo apt-get upgrade showed it was libgksu2-0, which then installed OK.
Lubuntu 13.04 had this problem with 3.8.0.28-generic (i386) a couple of weeks ago affecting 3 machines on my LAN.
A reboot found the MBR was corrupt on 1 of 3 machines, so maybe something else caused that.
It happened again with 3.8.0-29-generic this week.
sudo apt-get upgrade showed it was libgksu2-0, which then installed OK.