I have a really old Ubuntu (11.04) headless server installed on one of the client location.
The application created needed some update but unfortunately the Git was not installed on the system.
While trying to apt-get install git, got 404 (HTTP not found) error. When tried to do an apt-get update lots of 404 errors were there as Ubuntu 11.04 has reached end of life.
A quick search on the Google and found that old releases of Ubuntu are hosted on old-releases.ubuntu.com server(s).
To use this server I have to manually update /etc/apt/sources.list file
In the source list file I have to replace all instances of archives.ubuntu.com & security.ubuntu.com with old-releases.ubuntu.com followed by apt-get update
Finally was able to install git on the target system from old-releases servers of Ubuntu.
Random Thoughts 