Occasionally while running an apt-get update, you might see messages such as this:

Code:
Fetched 188kB in 2s (90.0kB/s)                                                 
W: Failed to fetch http://all.repository.backtrack-linux.org/dists/revolution/testing/binary-i386/Packages.bz2  Hash Sum mismatch
E: Some index files failed to download, they have been ignored, or old ones used instead.
This happens due to a repository flush. This also usually means there are going to be updates or new tools very soon in the repo. Flushes do not take more than 10 minutes, so if you get this error, wait a bit, and try again.