I first noticed there was a problem with the sitemaps generated by Yoast SEO a few weeks ago, having submitted the main sitemap to Google Webmaster Tools, some of the second level sitemaps within that sitemap were not being located due to a bug in yoast. For example: Main sitemap - mysite.com/sitemap.xml Second level category sitemaps - mysite.com/category1.xml The plugin was omitting the domain on the second level sitemaps causing sitemaps to be submitted with an incorrect path and webmaster tools was showing over 1400 url errors for my site. I posted on their support forum but was ignored for several weeks. Have since deactivated the sitemap portion of Yoast and re-installed Google Sitemaps plugin, webmaster tools now shows no errors having submitted the sitemap from Google Sitemaps plugin. May be worth checking your own sites if you use this plugin. It may not affect you as it was only happening on certain categories for me but it's definitely worth double checking in case it's affecting your user experience or rankings.