Comment 28 for bug 554079

Just happened to me as well (stuck at 75% complete). However I am able to ssh into the machine. This happened after I put an previous karmic xorg.conf back into /etc/X11/ to test an nvidia-96 problem (#523108 & #539196) and rebooted.

2.6.32-19-generic (updated as of this morning)
$ apt-cache policy e2fsprogs
e2fsprogs:
  Installed: 1.41.11-1ubuntu1
  Candidate: 1.41.11-1ubuntu1
$ apt-cache policy plymouth
plymouth:
  Installed: 0.8.1-4ubuntu1
  Candidate: 0.8.1-4ubuntu1
$ apt-cache policy mountall
mountall:
  Installed: 2.10
  Candidate: 2.10
$ apt-cache policy nvidia-96
nvidia-96:
  Installed: 96.43.14-0ubuntu11
  Candidate: 96.43.14-0ubuntu11

No instance of fsck running. Nothing obvious in dmseg, xorg.0.log, nothing new in /var/log/fsck.
Tail of .xsession-errors shows:
** (gnome-panel:1870): WARNING **: Failed to send buffer

** (gnome-panel:1870): WARNING **: Failed to send buffer
The application 'gnome-panel' lost its connection to the display :0.0;
most likely the X server was shut down or you killed/destroyed
the application.

so I'm thinking it may be related to the karmic xorg.conf file that I used. I'll try rebooting to see if any change (leaving the xorg.conf file in place).