With the advent of HTTP 1.1, came partial downloads.
Although partial downloads were originally intended to be able to resume interrupted downloads, they are mostly used in download manager software. These download managers retrieve big files by opening several connections at once and retrieving different pieces of the file in parallel.
This "cheating" places an unwanted burden on the network and on the Internet in general, and puts other users at a disadvantage: if five connections are opened by one user to one file, downloading will be five times faster for him, but five times slower for everybody else.
To prevent this from happening, only one download can occur from one IP address to any given file. Other connections to the same resource will be denied with a 503 - Service temporary unavailable error.
You should only see this error if you're using download manager software. If your connection is done through a Network Address Translation (or NAT) router, or you're behind a proxy server, you could also get this error if somebody else within your organization accesses the same file. In that case, please try again later.
In case of problems, please email the webmaster.