Status code 000 in the logs

[monkeybrain]

What does status code 000 mean in the Nginx logs? I have the following in
the config:

        location  =  /get-img.pl
        {
            limit_req  zone=slow  burst=10;
            proxy_pass  http://service_perl;
        }

get-img.pl generates an image, writes it to disk and then returns
"X-Accel-Redirect" to the image. Everything seems to be working just fine, I
see delayed responses because of limit_req in the logs (with code 200).
Sometimes there are responses with code 503 for "greedy" clients that exceed
the burst=10 parameter. All as expected. 

However, occasionally I see a bunch of requests (around 5 to 20 within a few
seconds of each other) with status code 000 and 0 for the body size in the
logs. They are always from the same IP address for the entire bunch, so I'm
guessing it's requests that already went through internal "X-Accel-Redirect"
redirect and then something happened. Connection aborted? Something else?
Why not status code 503? That's where you come in with a helpful
explanation. :)

In case it is relevant:
nginx version: nginx/1.4.2
built by gcc 4.1.2 20080704 (Red Hat 4.1.2-54)
TLS SNI support disabled
configure arguments: --prefix=/etc/nginx --sbin-path=/usr/sbin/nginx
--conf-path=/etc/nginx/nginx.conf --error-log-path=/var/log/nginx/error.log
--http-log-path=/var/log/nginx/access.log --pid-path=/var/run/nginx.pid
--lock-path=/var/run/nginx.lock
--http-client-body-temp-path=/var/cache/nginx/client_temp
--http-proxy-temp-path=/var/cache/nginx/proxy_temp
--http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp
--http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp
--http-scgi-temp-path=/var/cache/nginx/scgi_temp --user=nginx --group=nginx
--with-http_ssl_module --with-http_realip_module --with-http_addition_module
--with-http_sub_module --with-http_dav_module --with-http_flv_module
--with-http_mp4_module --with-http_gunzip_module
--with-http_gzip_static_module --with-http_random_index_module
--with-http_secure_link_module --with-http_stub_status_module --with-mail
--with-mail_ssl_module --with-file-aio --with-ipv6 --with-cc-opt='-O2 -g
-pipe -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector
--param=ssp-buffer-size=4 -m64 -mtune=generic'

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,242893,242893#msg-242893