Google specific error 500

A new unexplained challenge. Google Webmaster tools showed all pages of a site under my care with a "Network Unreachable" error. The site had recently undergone a transition from a single languge site to a multilingual site. Viewing the site in various browsers across varying platforms did not appear to cause any error, perhaps when google had crawled it, there had been a server problem, and it was unreachable only at that time.
Under careful watch, the problem persisted, and it turns out it came from a piece of code which attempted to detect a users language settings from the browser when the user first visited the website. The file had a reference to Browser Languages - and unfortunately, had assumed that a language, any language, reference would be returned (even if that language was not a language handled by the site) - and began its checks as to whether it should default to a predefined language or attempt to serve the site in the requested language from that point.
I would never have found this error without the URI Valet - a most useful tool, for checking your site server headers and allowing you to render the site as it would appear to google, or to a number of other bots and browsers. URI Valet is an invaluable tool for all internet marketers, site owners and developers - designed it so it is easy to understand and allows you to quickly determine if certain potential technical issues exist that could, or are, causing problems with your URL or domain name.