Comment 1 for bug 1533690

[apt_update]/returns (notice): W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-security/main/binary-amd64/Packages Hash Sum mismatch
[apt_update]/returns (notice):
[apt_update]/returns (notice): W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/trusty-security/universe/binary-amd64/Packages Hash Sum mismatch
[apt_update]/returns (notice): E: Some index files failed to download. They have been ignored, or old ones used instead.
[apt_update] (err): Failed to call refresh: /usr/bin/apt-get update returned 100 instead of one of [0]
[apt_update] (err): /usr/bin/apt-get update returned 100 instead of one of [0]

It's either a network issue, or Package(.gz/.bz2) files have been updated on the mirror (a race condition issue).