I tried this plug-in and it seemed to work great at first. I added a word list and I wanted to test it by editing a post that included that word. It seemed to do everything as expected, but I clicked on an older post that included the same word and this time I got a 404 error. It seems to only work from that point forward and any older posts that contain one of the stopwords will break with a 404 error.
Any idea how to run this without breaking old posts?
http://wordpress.org/extend/plugins/remove-stopwords-from-slug/