Let's say you're using Ubuntu 13.04 (Raring Ringtail, released in April 2013) and it just went End-of-Life on you, because it's supported for only 6 months, and the deprecated packages are taken down after 12 months.
You'll probably figure this out the hard way. When you run sudo apt-get update, it will eventually report these errors:
Ign http://archive.ubuntu.com raring-updates/universe Sources/DiffIndex
Err http://security.ubuntu.com raring-security/main Sources
404 Not Found [IP: 91.189.91.15 80]
Err http://security.ubuntu.com raring-security/universe Sources
404 Not Found [IP: 91.189.91.15 80]