This error message is given by MacPar Deluxe on the Mac, and possibly by a few other RAR programs. It means that something went wrong in the downloading of your RAR files. This is a common problem when downloading RAR files from Usenet, or downloading from other sources when the RAR files originated on Usenet.
In most cases, you can fix your RAR files by downloading the associated PAR files. If you were downloading with an NZB file, look in your NZB program’s settings and see if it defaults to skipping PAR files. If so, tell it to download the PARs, and re-download the NZB.
If for some reason it still won’t work, try again with SuperNZB, which can handle many different types of error conditions.