I am getting these kind of errors:
2014/05/24 11:49:06 [error] 8376#0: *54031 upstream sent too big header while reading response header from upstream
This is still the highest SO-question on Google when searching for this error, so let's bump it.
When getting this error and not wanting to deep-dive into the NGINX settings immediately, you might want to check your outputs to the debug console. In my case I was outputting loads of text to the FirePHP / Chromelogger console, and since this is all sent as a header, it was causing the overflow.
It might not be needed to change the webserver settings if this error is caused by just sending insane amounts of log messages.