Persistent hash sum mismatch in aptitude

[Hendrik Boom <hendrik@topoi.pooq.com>]

Whenever I do an
   aptitude update
it seems to work OK, except for a series of messages at the end:

W: Failed to fetch http://ftp.ca.debian.org/debian/dists/testing/contrib/
source/SourcesIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.ca.debian.org/debian/dists/testing/contrib/
binary-i386/PackagesIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.ca.debian.org/debian/dists/testing/contrib/
i18n/Translation-enIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.ca.debian.org/debian/dists/testing/non-free/
i18n/Translation-enIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.us.debian.org/debian/dists/testing/contrib/
source/SourcesIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.us.debian.org/debian/dists/testing/contrib/
binary-i386/PackagesIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.us.debian.org/debian/dists/testing/contrib/
i18n/Translation-enIndex: Hash Sum mismatch
W: Failed to fetch http://ftp.us.debian.org/debian/dists/testing/non-free/
i18n/Translation-enIndex: Hash Sum mismatch
E: Some index files failed to download. They have been ignored, or old 
ones used instead.
E: Couldn't rebuild package cache

Current status: 429 updates [+14], 15048 new [+7].

Notice that this happens with *two* mirrors.

Is there something I should do to stop this?

Is it even something to worry about?  My subsequent safe-upgrades seem to 
work just fine.

-- hendrik