Конфиг стандартный из коробки, едиственное, что поменено это число воркеров. Покурив логи, там 502 была только на маках и айфонах.
nginx.conf
sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        # server_tokens off;
 server_names_hash_bucket_size 64;
        # server_name_in_redirect off;
          fastcgi_buffers 8 16k;
        fastcgi_buffer_size 32k;
        fastcgi_connect_timeout 300;
        fastcgi_send_timeout 300;
        fastcgi_read_timeout 300;
        include /etc/nginx/mime.types;
        default_type application/octet-stream;
        ##
        # Logging Settings
        ##
        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;
        ##
        # Gzip Settings
        ##
        gzip on;
        gzip_disable "msie6";
        gzip_vary on;
        gzip_proxied any;
        gzip_comp_level 9;
        gzip_buffers 16 8k;
        gzip_http_version 1.1;
        gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;
location ~ \.php$ {
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                fastcgi_pass unix:/var/run/php5-fpm.sock;
                fastcgi_index index.php;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                fastcgi_param HTTPS on;
                fastcgi_param  HTTP_X_REAL_IP   $remote_addr;
                include fastcgi_params;
        }