We had the WP Retina plugin work on files uploaded and we were getting server 500 errors. It appears theres an php error since we updated to v3.6 of WordPress. As soon s we disable the plugin uploads are fine again.
Here is the error.
2013/08/12 04:20:18 [error] 3596#0: *31794 FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to undefined function wr2x_get_attachment_id() in /var/www/vhosts/i-16852366/wp-content/plugins/wp-retina-2x/wp-retina-2x.php on line 382" while reading response header from upstream, client: unix:5acbf9, server: _, request: "POST /upload-mixtape/ HTTP/1.0", upstream: "fastcgi://unix:/var/run/php-fpm.sock:", host: "themixtapesite.com", referrer: "http://themixtapesite.com/upload-mixtape/"