I am getting this a lot as well. My primary source is the local mirror which takes longer to update than the servers under http://http.us.debian.org
My feeling for this is that the index files are uploaded before the actual package files. This would mean that when you do an update you end up with pointers to files that are yet to be uploaded to the servers. I have not analysed it enough to be able to provide proof, and as such have not put together a bug report. Sometimes I get pointed to different servers under http://http.us.debian.org some of which give better results than others. Assumedly these servers are updated earlier in the mirror distribution cycle regards Mark Karsten said with exemplary erudtion even if the txt was in a mime attachment: I'm getting problems with 404 codes on apt-get dist-upgrade or install attempts, in particular, from http.us.debian.org. <snip>
