This may not be a WordPress thing at all, but I've never seen it before and it's happening to a WordPress site we're completing. I use Xenu for checking sites for broken links, images etc. When I run it over our dev site I see loads of 404s for what are valid URLs, but with /yes and /no appended to them. i.e.
http://mysite/2014/04/my-fascinating-post/yes
http://mysite/2014/04/my-fascinating-post/no
Has anyone seen anything like this before?!
Thanks,
Toby