There is no 499 http code in rfc. Nginx defines 499 code itself.
When a client sent a request, and closed the connection without waiting for
the response, a 499 code occurs. If there’re a lot of 499s in your
access_log, it’s mostly caused by the slow back-ends (too slow for your
users to wait). You may have to optimize your website performance.
http://forum.nginx.org/read.php?2,213789,213794#msg-213794