I see unusual amount of 408 errors from access log

[xiaowl]

/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:01:20 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:10:57 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:16:55 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:18:31 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:21:54 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:25:57 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:33:43 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:36:02 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:36:21 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:49:05 +0000]
"-" 408 0 "-" "-"
/var/log/nginx/nost_access.log.1:x.x.x.x - - [10/Jun/2013:09:55:31 +0000]
"-" 408 0 "-" "-"

As you can see, the log doesn't contain any information about request, like
request method and request uri. A normal access log should like this:

/var/log/nginx/nost_access.log.1:x.x.x.x - - [09/Jun/2013:22:49:58 +0000]
"POST /some/resource/3984/upload/ HTTP/1.1" 408 0 "-"
"APIClient-Android-16#v0.8.6"

My question is, why some access log entries have no request method/uri? I
guess the 408 occurs in a early phase when the request entry is not
processed yet. Since I'm enable HTTPS, is that can be a reason about this
error?

My Nginx version is 1.2.1.

Thanks.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,240003,240003#msg-240003