I have always had this issue, not sure where to begin to fix it because I don't know how wordpress deals with this. If someone tries a non-existing url at my website like this:
http://www.wetcircuit.com/not-a-page
I don't get a 404 error or a "not found" page, what I get is every post in every category (as opposed to the front page which is set to show only certain categories. At one time, I created a hidden category, but because of this issue I had to abandon it -- they all showed! Somehow this issue gets around the plugins.)
If you can point me to the correct keywords? Or maybe help with a link to the docs? Is this an htaccess issue or maybe I am missing a php in my theme? Sorry, don't understand enough to start the answer on my own....