If you are using SEO Framework for WordPress and NGINX you may find robots.txt and sitemap.xml give a 404 error. The following rewrite rules can be added to fix the issue (at least as of SEO Framework 4.1.3). Be sure to comment out or remove any other rewrite rules, especially those with a “last” directive, that match against robots.txt and sitemap.xml.
rewrite ^/robots\.txt$ /index.php?robots=1 last;
rewrite ^/sitemap\.xml$ /index.php?the_seo_framework_sitemap=xml last;
rewrite ^/sitemap\.xsl$ /index.php?the_seo_framework_sitemap=xsl last;