News Courtesy of Yoast.com:
In our major Yoast SEO 7.0 update, there was a bug concerning attachment URLs. We quickly resolved the bug, but some people have suffered anyhow (because they updated before our patch). This post serves both as a warning and an apology. We want to ask all of you to check whether your settings for the redirect of the attachment URLs are correct. And, for those of you who suffered from a decrease in rankings because of incorrect settings, we offer a solution that Google has OKed as well.
The impact of the bug
For some sites our bug might have a truly bad impact. In Twitter and Facebook discussions I’ve had, I’ve been shown sites that had the number of indexed URLs on their site quintupled, without adding any content. Because with that setting being “No” XML sitemaps was enabled for attachments. As a result of that, lots and lots of attachment URLs got into Google’s index. Some of those sites are now suffering from Panda-like problems. The problem will be specifically big if you have a lot of pictures on your website and few high quality content-pages. In these cases, Google will think you’ve created a lot of ‘thin content’ pages all of a sudden.
The vast majority of the websites running Yoast SEO probably hasn’t suffered at all. Still, we messed up. I myself, am sorry. More so than normal, because I came up with and coded this change myself…
Ryan’s Take 
As an avid user of yoast on all of the WordPress sites I’ve built, I’ve been pretty impressed with just how efficient and organized the plugin is. When I read the news about this bug, it was shocking on a few different levels. For one, the Yoast team has always been thorough with the features they offer and introduce. Having an option inadvertently reset to a potentially harmful setting just didn’t seem possible. Alas, accidents happen.
Another observation of mine was how Yoast handled the situation publically. They could’ve remained mum on the issue or flat out denied it. Instead, here they are falling on the sword. I’m sure there are some who have lost faith in the plugin. I’m not one of those people. It’s not like they maliciously made a change to cause damage. Also, as stated by Google, if these attachment URLS are getting indexed, then your website might have bigger problems. I actually had it set to no on precisiondigital-llc.com. From what I can tell, it appears I haven’t been affected by this bug.
Is Yoast solely to blame for a rankings decline? I don’t believe so. As designers and developers, there is a responsibility to ensure a certain level of quality when it comes to content and indexing. Periodic checks for indexed pages should be done using the site:domainname.com query. You wouldn’t believe how many colleagues of mine have garbage content indexed. A lot of the times it is demo content from a theme. How embarrassing would it be if a client found a demo/dummy page on their website indexed in Google? The first thing that should be done before submitting your website to Google is to check the sitemap file. Make certain that only the posts, pages, and taxonomies you wish to be indexed are visible on the sitemap.
