A word about 404 errors
One of the most common crawling errors are 404 errors, which occur when somebody tries to access a page that does not exist (usually because the page has been deleted or the user clicked on a broken or incorrect link). Most of the time, 404 errors can be ignored. However, if you’re seeing a lot of traffic leading to a URL that 404s, check your site for broken links. More information about dealing with 404 errors.
http://support.google.com/webmasters/bin/static.py?hl=en&page=checklist.cs&tab=1095580
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