An error 503 is a proxy error - you are able to contact the machine from which you get your web resources, but it's unable to contact the machine 'behind' it. The majority of users browse 'directly' to the server, but a proportion will to to one server which in turn will ask another. The idea is that if you contact a local machine that's had a copy of the same static page / style sheet / image for someone else, it can then hand out a stored copy; it also works well for cases where you contact a machine that's (supposedly) serving a whole domain but it then passes on some of the queries. That's often done with Apache httpd and Tomcat to server Java
(see here if you want to learn more from me about this 
) but we do NOT do it for the Coffee Shop.
Conclusion ... one of the machines that your dozey device is contacting has to refer onwards and is probably running out of puff. For most sites, it would just give you an old cached version, but we're so dynamic that it has to ask every time in case there have been more posts and service changes even in the few minutes since a page was last called up.
