Comment 19 for bug 1436691

I have found some additional information on how this bug would be reproduced, let me explain:

If you visit this URL: http://cdimage.ubuntu.com/ubuntu-gnome/releases/15.04/ , there are four alpha/beta releases of 15.04 in that directory: alpha-1, alpha-2, beta-1 and beta-2.

Now check the manifest file for alpha-1, alpha-2 and/or beta-1 (but not beta-2), for example:
http://cdimage.ubuntu.com/ubuntu-gnome/releases/15.04/beta-1/ubuntu-gnome-15.04-beta1-desktop-amd64.manifest

You will see that these packages related to systemd and upstart were installed by default:

systemd 219-3ubuntu1
systemd-shim 9-1
upstart 1.13.2-0ubuntu7
upstart-bin 1.13.2-0ubuntu7

Notice that the systemd-sysv package is NOT installed by default in the manifest file? This means the binaries such as /sbin/init and /sbin/shutdown are only provided by the "upstart" package.

Now, in the latest version of upstart package, these binaries were moved to upstart-sysv and are no longer contained in the upstart package itself.

To sum it up, if you start with an alpha-1, alpha-2 and/or beta-1 build of Ubuntu 15.04, and from there, attempt to do a "apt-get update; apt-get dist-upgrade" (or "aptitude update; aptitude upgrade"), it will cause the system to lose those essential binaries and can no longer shutdown/reboot.