Ok I got Google message in Webmaster Tool for my website: "http://www.example.com/: Possible outages" Detail message is as:
Dear owner or webmaster of http://www.example.com/
While crawling your site, we have noticed an increase in the number of transient soft 404 errors around 2012-07-18 02:00 UTC (London, Dublin, Edinburgh). Your site may have experienced outages. These issues may have been resolved. Here are some sample pages that resulted in soft 404 errors:
I don't know what's going wrong with my website.
[edited by: tedster at 1:27 pm (utc) on Jul 25, 2012] [edit reason] switch to example.com [/edit]
It sounds like there are two issues being pointed to here. The first one was temporary website outages that may not have continued. In other words, the sample pages that were mentioned may now be working fine. If you cannot duplicate the problem - even using the Fetch as Googlebot tool - then there's nothing for you to fix.
And the second issue seems to be that your server responded with a "soft 404" - a 404 message but a 200 OK http header. Here are some Google references about the soft 404 problem.
This message generally appears when your site is experiencing downtime during Google crawl and in place of 503 you are giving a 200 OK response with a under-construction page or similar (across all URL's). If you can change the response code to 503 during downtime the transient 404 message should be gone.