In Ubuntu 15.04, I'm getting this after doing sudo apt-get update:
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/binary-amd64/Packages 404 Not Found
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/vivid/main/binary-i386/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
Go to this link: http://extras.ubuntu.com/ubuntu/dists/
As it says, vivid is not there. Did they move it? Will they move it back? How do I repair?
Extras is being retired. It'll hang around until 14.04 runs out of support in 2019 but it won't have newer distributions added to it.
What you are seeing is the result of upgrading too early (Vivid isn't realeased yet) so the upgrader script you used didn't remove it from your sources, (per this bug).
To fix, simply edit the relevant lines out of your
/etc/apt/sources.list
.