Common SEO mistakes include using keywords excessively in titles, having high keyword densities, lacking an XML sitemap, not including keywords in URLs or using unique meta data and descriptions for each page. Other mistakes are overusing H1 tags, using the same anchor text for all links, not setting up 301 redirects, lacking keywords in content, focusing too much on meta tags, not providing HTML alternatives to Flash sites, engaging in keyword spamming, choosing a crawler-unfriendly CMS, and not promoting the site.