Bad requests with 0-byte response

[Jean-Baptiste Quenot]

Hello,

I'm using nginx 0.6.35 with syslog patch, and have a few POST requests
everyday that are aborted by nginx, with no error message being
reported to the error log.  The status code returned is 400, and there
is no single byte written by nginx.  Those requests are supposed to be
forwarded to a FastCGI application, but the latter is never reached,
connection is aborted right away.

How to diagnose what's going wrong?  Is there a way to trace aborted
requests, or requests to a specific location?  If there is no such
possibility, I'll probably rebuild nginx with the debug configure
switch turned on, but I'm a bit concerned to run it in production,
with thousands of requests every second.

Thanks in advance,
-- 
Jean-Baptiste Quenot