If you download 7 times or so, always get a mismatching md5sum but so that they are same for all the 7 downloads, then you can be pretty sure it's a broken image (it's unprobable that the download would become crap exactly the same way each time). If the md5sums vary, then it's in the connection or somewhere.
I've seen some (not Debian) mismatching checksums after I've downloaded some iso files, sometimes trough http, sometimes trough ftp, never trough torrent (because my ISP is naughty and has, in some bizarre way, restricted downloading torrents faster than 5k/s which is modem speed). In every case I've bothered to check it has not been a bad image on the server, just bad download. Sadly, no matter how good tcp/ip is, it's not perfect.
It's good we have md5sums and sha1sums and so on, and it's good that we have different tools to check they're genuine, but I just hope they were faster
even on modern computers it takes dozens of seconds for big files, even longer.
