We have an old VM (Ubuntu 14.10) that we just have running – it does a very specific job, isn’t connected to anything important and just shugs along. But because of a dependency issue with external software, we needed to install a new library to it – and because of dependencies when we first set up the VM, Ubuntu was the distro selected.
Sadly all the old URLs for apt-get in sources.list had stopped working, as the mirrors no longer had that specific Ubuntu (utopic) version available.
Luckily – after a bit of using our old friend Google – I found old-releases.ubuntu.com. This is also available as an archive for content through APT, so if you prefix your old addresses with old-releases.ubuntu.com instead of whatever mirror you’re used to fetching images from, you can get last version of the packages made available when you first set up your distro.
Saved the day!