Comment 4 for bug 870832

Well, one problem in the ubiquity-dm code is that it has no SIGTERM signal handler. So anything that causes ubiquity-dm to die by signal (i.e., anything that causes upstart to want to stop the job, such as a manual call to 'service ubiquity stop' or a runlevel change) does not clean up the child processes... such as the X server.

This doesn't explain the behavior at the end of an install, because in that case ubiquity-dm does try to shut down the X server before calling reboot. It might still be related to the race condition between ubiquity-dm shutting down (thus no longer blocking the kdm job and allowing it to try to start an X server) and the runlevel event being propagated by the 'reboot' call. If so, it might be possible to address this by having ubiquity-dm not exit in the reboot case but instead wait to receive SIGTERM.