WordPress version: 3.5.1
WordPress SEO version: 1.4.7
I installed this plug-in: BAW Post Views Count
http://www.boiteaweb.fr/baw-post-views-count-3232.html
When I activated it, all of my posts that I had optimized with Yoast suddenly went from "OK" status to red "Bad" status even though nothing changes under page analysis.
When I deactivate that plug-in all posts go back to their previous status. I'm not sure what the error might be, reporting the issue to that developer as well.