1. Install Ubuntu 12.04, enable the 'universe' repository
2. sudo apt-get install jenkins
3. Add the upstream deb repository at 'http://pkg.jenkins-ci.org/debian' to sources.list, upgrade (dealing with JENKINS-18798 along the way)
4. Reboot (this may happen a long time after step 3

Expected behavior: Jenkins is up and running

Actual behavior: no it's not.

Details:

The Ubuntu jenkins package uses upstart and installs an /etc/init/jenkins.conf. When you upgrade to the upstream package (which doesn't ship this file), dpkg leaves the /etc/init/jenkins.conf conffile in place. So next time you boot, Upstart will try to use the Ubuntu upstart config instead of the upstream init.d file. The upstart config file tries to run /usr/share/jenkins/bin/maintain-plugins.sh before starting Java, but that file doesn't exist, so the job fails.

Workaround:

rm /etc/init/jenkins.conf
update-rc.d jenkins defaults
service jenkins start