Page is a not externally linkable
-- Google SEO News and Discussion
---- Pages showing up in WMT as 'not found' don't actually exist
g1smd - 11:26 pm on Sep 8, 2012 (gmt 0)Thread source:: http://www.webmasterworld.com/google/4492682.htm
Sounds like some botched relative internal linking somewhere on the site.
Run Xenu LinkSleuth over the site and look very carefully at the reports.
Brought to you by WebmasterWorld: http://www.webmasterworld.com