Why would a 503 Error occur in one location but not in another?

Why would a '503 - Service Temporarily Unavailable' message appear for a website in one place, but load perfectly well in another? Other websites are loading fine where the user is recei开发者_开发技巧ving a 503 error.

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If you're behind a proxy server or firewall (e.g. on a company's internal network, or in a country that controls network access), the proxy may not be working correctly, or may be too busy and will return that error.

See: http://en.wikipedia.org/wiki/Proxy_server

If you have access to the server's log, you can test this case by reviewing the log. If requests getting 503 errors are never showing up in the log, it's likely an error at the intermediate proxy.