I just wanted to make an upgrade "on the run" to get latest security patches and fixes onto my laptop. But, unfortunately, similar to the last time I thought I could upgrade an Ubuntu system in a matter of minutes(just to realize that in the end, mysql wasn't going to start anymore), something broke:

The ntp package wasn't installable, hanging on errors like this:

E: Sub-process /usr/bin/dpkg returned an error code (1)
A package failed to install. Trying to recover:
Setting up ntp (1:4.2.4p4+dfsg-3ubuntu2.1) ...
usage: update-rc.d [-n] [-f] <basename> remove
       update-rc.d [-n] <basename> defaults [NN | sNN kNN]
       update-rc.d [-n] <basename> start|stop NN runlvl [runlvl] [...] .
        -n: not really
        -f: force
dpkg: error processing ntp (--configure):
 subprocess post-installation script returned error exit status 1
Errors were encountered while processing:
 ntp

Some research helps me finding that the problem obviously comes from line 47 in the postinst script:

 update-rc.d ntp multiuser 23 >/dev/null

The multiuser keyword seems unknown to update-rc.d - and that is no secret to Ubuntu developers!

Replacing multiuser with "defaults" - I did it manually now by editing /var/lib/dpkg/info/ntp.postinst - lets me install the package properly, or, in this case now, fix the broken install with

apt-get install -f

Reported/added information to https://bugs.launchpad.net/ubuntu/+source/ntp/+bug/317128