Comment 26 for bug 932088

The 403s are in fact weird, but they are explainable. The reason for them is that the 'Sources', and 'Packages' files do not exist in uncompressed form on S3 *or* on the *.archive.ubuntu.com mirrors (see http://us.archive.ubuntu.com/ubuntu/dists/natty-updates/universe/source/ )

The reason you see the 403 is that apt got bad data for the Sources.bz2 (or Sources.gz) and decided to try the 'Sources' as a fallback. It does unfortunately hide the real issue quite well :)