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Thanks for the question. It isn't uncommon for there to be strange 404 errors in Search Console with little information/bad information. They are working hard to improve this, but I wouldn't take everything you see there as set-in-stone. This doesn't sound like a negative SEO tactic. I would just mark them all as fixed, and see if they appear again in about a week. If they do, I'd make sure they are actually served as 4xx status and not worry too much about it. If you want to do more digging... Some ideas of where you could look further Logs logs logs. This will be the ultimate truth - you will be able to see whether or not GoogleBot is actually hitting those URLs. It could be something weird happening with a plugin of yours that generates those URLs (particularly on Wordpress). Perhaps you have a filtering system setup that generates these URLs? If you have a search function on the site, sometimes weird URLs can be generated through that. Do the URLs come-up when you crawl the site at all? Just a few ideas!

Well, after submitting multiple temporary sitemaps and having Google index them, our duplicate counts dropped back to pre-event levels. However, our rankings haven't improved at all. Actually, if anything, they dropped even further. At this point it's really starting to look like this is a hit from Panda 4.1 and that we had our URLs change was merely a coincidence. From the looks of it, Google is now marking our site as a low quality site. Now that we know about such a thing, we definitely experienced a 'sinister surge' prior to disaster striking. Since we've never engaged in any bad behavior on the site and we've always followed google's best practice advice, we're currently at a loss of what could be the reason that we're hit that way. Our content is fresh and high quality (arguably the highest quality in our domain), we have a very decent link profile according to MajesticSEO, so for now, no clue about what's going on really. Attached is the site's impressions and clicks graph from Webmaster Tools. PAgd8q6.png