Not like there are any good SEO mistakes, but at least some mistakes are learning lessons you can recover from. A dumb SEO mistake is one that puts your site out of commission, with no hope of recovery. For instance, think of eHow and their constant tendency to publish thin content. Remember how hard they got smacked during the first Panda algorithm update? Of course, that site has since recovered but it took countless years and revisions to the site to make it more relevant and dejunk all that spam and who can say if they are nearly as popular as before.
With that in mind, what's one really dumb SEO mistake that you made for your own sites or that you know of having been made on another site?