I received the following email from Google:
Google detected a significant increase in the number of URLs that return a 404 (Page Not Found) error. Investigating these errors and fixing them where appropriate ensures that Google can successfully crawl your site’s pages.
But what is strange is that the missing pages are all like this:
[link removed as its pointing to malware now]
The “regular” pages are all okay.
I’m using Day and name as my permalink structure and the W3 Total Cache Plugin. The sitemap is being generated by Yoast WordPress SEO. I also have some others plugins installed.
What I think that is happing is that Google is indexing my cache somehow and when the cache is cleaned, the 404 error appears. I don’t know if Google found those cached pages by crawling or using the sitemap.
My blog is http://mafagrafos.net in case anyone needs to look at something.
This issue has been raised in the Google Webmaster Forum and Google have acknowledged it and are working on a solution.
https://productforums.google.com/forum/#!topic/webmasters/zvez-eib0Ao/discussion
That would be the best place to add your input as it may help Google resolve it.