Nginx 1.6 under load gives a ton of error 403

[dfumagalli]

Domain anonymized sample error log: see the first entry returns 200, the
next 403 on the same page.

2.139.79.51 - - [06/May/2014:08:13:23 +0000] "GET /flowers/love HTTP/1.1"
200 62
55 "http://dev.domain.com/flowers/friendship" "Mozilla/5.0 (Windows NT 6.3;
WOW64; rv:28.0) Gecko/20100101 Firefox/28.0"
2.139.79.51 - - [06/May/2014:08:13:28 +0000] "GET / HTTP/1.1" 403 134
"http://de
v.domain.com/flowers/love" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0)
Gec
ko/20100101 Firefox/28.0"
2.139.79.51 - - [06/May/2014:08:15:01 +0000] "GET / HTTP/1.1" 403 134
"http://de
v.domain.com/flowers/love" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0)
Gec
ko/20100101 Firefox/28.0"
2.139.79.51 - - [06/May/2014:08:17:31 +0000] "GET / HTTP/1.1" 403 134
"http://de
v.domain.com/flowers/love" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0)
Gec
ko/20100101 Firefox/28.0"
2.139.79.51 - - [06/May/2014:08:17:52 +0000] "GET / HTTP/1.1" 403 134
"http://de
v.domain.com/flowers/love" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0)
Gec
ko/20100101 Firefox/28.0"

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249857,249859#msg-249859