Alert "already header sent" is printed in error.log

Hi, I use nginx-1.8.0 as a reverse proxy.
When I set upt the configuration and ran it, I got the following errors.
in debug.log file
2015/08/20 17:17:25 [debug] 12180#0: *354785 writev: -1 of 333
2015/08/20 17:17:25 [info] 12180#0: *354785 writev() failed (104: Connection reset by peer), client: xxx.xxx.xxx, server: yyy.yyy.com, request: "GET /web/css/review.css?v=1gh-ey5j1q HTTP/1.1", host: "yyy.yyy.com"
2015/08/20 17:17:25 [debug] 12180#0: *354786 writev: -1 of 333
2015/08/20 17:17:25 [info] 12180#0: *354786 writev() failed (104: Connection reset by peer), client: xxx.xxx.xxx, server: yyy.yyy.com, request: "GET /web/css/lightbox.css?v=25p-2ojfz3 HTTP/1.1", host: "yyy.yyy.com"
2015/08/20 17:17:36 [debug] 12182#0: *355174 writev: -1 of 334
2015/08/20 17:17:36 [info] 12182#0: *355174 writev() failed (104: Connection reset by peer), client: xxx.xxx.xxx, server: yyy.yyy.com, request: "GET /web/css/deal.css?v=hoh-jk4kpp HTTP/1.1", host: "yyy.yyy.com"

in error.log file
2015/08/20 17:17:25 [alert] 12180#0: *354785 header already sent, client: xxx.xxx.xxx, server: yyy.yyy.com, request: "GET /web/css/review.css?v=1gh-ey5j1q HTTP/1.1", host: " yyy.yyy.com"
2015/08/20 17:17:25 [alert] 12180#0: *354786 header already sent, client: xxx.xxx.xxx, server: yyy.yyy.com, request: "GET /web/css/lightbox.css?v=25p-2ojfz3 HTTP/1.1", host: " yyy.yyy.com"
2015/08/20 17:17:36 [alert] 12180#0: *355174 header already sent, client: xxx.xxx.xxx, server: yyy.yyy.com, request: "GET /web/css/deal.css?v=hoh-jk4kpp HTTP/1.1", host: " yyy.yyy.com"

No error occurred in client.
And nginx-1.6.3 doesn't print these logs.
Has anybody faced to this case?