When you use the Mandriva GUI to apply updates it automatically updates the media database, however when the mirror sites are being upodated they can sometimes get out of sync. This is because the updates are not atomic. In other words, the system does not update the list of files present and the actual directory contents at the exact same moment. For this reason, you will sometimes see this error. It will automagically go away on its own if you wait a bit and try again when this is the problem.
It is also possible that the Mandriva team makes an error, and the files on the mirror and the ones in the database do not match for this reason. The solution in this case is to wait a bit longer ;-)
(i.e. The Mandriva team will fix the error)
When one is not using the GUI tools (which are merely a wrapper for urpmi and urpmq scripts, etc) you can do, as root:
The above will update all the data for all of the repositories you have configured.