Got this today when trying to upgrade the software on my Ubuntu 12.04 Server:

# apt-get upgrade

Reading package lists... Done

Building dependency tree   

Reading state information... Done

0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

2 not fully installed or removed.

After this operation, 0 B of additional disk space will be used.

Do you want to continue [Y/n]?

Setting up dbus (1.4.18-1ubuntu1.5) ...

adduser: The user `messagebus' already exists. Exiting.

dpkg: error processing dbus (--configure):

 subprocess installed post-installation script returned error exit status 1

dpkg: dependency problems prevent configuration of dbus-x11:

 dbus-x11 depends on dbus; however:

  Package dbus is not configured yet.

dpkg: error processing dbus-x11 (--configure):

 dependency problems - leaving unconfigured

No apport report written because the error message indicates its a followup error from a previous failure.

                                                                                                          Errors were encountered while processing:

 dbus

 dbus-x11

E: Sub-process /usr/bin/dpkg returned an error code (1)

I'm tempted to delete the user messagebus and retry. Would that be a good way to fix this? Or do you have other suggestions?