You have a comprehensive site with dozens of landing pages, scores of main pages and category pages, and hundreds of weekly blog posts aimed at increasing traffic and SEO. The vast majority of all of this works flawlessly. The pages load correctly, the links work and traffic flows as it should. But, unknown to you, you also have a single page that generates a 404 error. The page cannot be found. And that page is likely costing you the majority of its traffic.