We had a problem when installing a bespoke Ubuntu 10.04 Server LTS setup via shell scripts we had written. Some of the packages were not found. This mainly occurred on virtual dedicated servers. We had previously installed on more than 20 normal dedicated server ok.
Turns out that the /etc/apt/sources.list had only a very limited list on the ones that didn't work. I copied the file from a normal server that was ok, and re-tried our scripts. That solved the problem.
Not too sure if you could build up other problems doing this, so always save a copy of the original file somewhere safe before changing.
Examples follow after the break...
