Strange 499 response code?

my server use nginx+php-fpm ,nginx as reverse proxy server, but our
access log got a lot of 499 response code,the document say that 499
means "client abort the connection “,i think the the reason cause "
client abort connection” comes from two: the first is client is in weak
network environment, client request can’t be delivered successfully, and
client has canceled the request. the second is php-fpm is too
busy,such as
for a long time nginx can’t get the response from upstream
,and the client canceled the request.

my question is  the first reason can be accepted . but the second 

since i checked the nginx access log, when there is 499 response code
nginx access log file ,there is always no “upstream_response_time”
value in it, seems the php-fpm is not busy and can server it
. here is some sample 499 nginx access log and some explanation: - 31/Oct/2012:13:09:34 +0800 POST
HTTP/1.1 499 0($body_bytes_sent) - Dalvik/1.4.0
U; Android 2.3.4; LT18i Build/4.0.2.A.0.62) 21($content_length)
2.448($request_time) -($upstream_response_time) - - - 31/Oct/2012:13:09:34 +0800 POST
HTTP/1.1 499 0 - Dalvik/1.2.0 (Linux; U; Android
2.2.1; GT-S5670 Build/FROYO) 44 10.034 -

Posted at Nginx Forum:,232418,232418#msg-232418