Seeing as the error happened on the same place each time, it might sound like you were getting errors that didn't _always_ get caught by the checksum algorithm (i.e the error "happened" to match the checksum) (maybe).
Sounds like another good reason to do separate checksumming of files after download (read: sha1/md5).
Needless to say the whole operation made the corruption checksum-neutral from the viewpoint of the "real" checksum - so it was not caught by the TCP checksum, as a result the files were sometimes silently corrupted.
Took a lot of work to catch and debug, the symptom was that Gentoo's packages appeared to be corrupt and failed the SHA1 checksum check.
So, indeed, cryptographic checksumming of files is a good thing.