Help decipher error message "upstream prematurely closed connection while reading response header fr

Anyone got any idea what this error means?

2008/05/02 11:38:31 [error] 5505#0: *357728 upstream prematurely
closed connection while reading response header from upstream, client:
69.212.152.80, server: tanga.com, URL:
“/puzzles/847-5-1-2008?comments_page=1#comment_188860”, upstream:
http://10.7.217.165:9013/puzzles/847-5-1-2008?comments_page=1#comment_188860”,
host: “www.tanga.com”, referrer:
http://www.tanga.com/comments/188860/edit

I’m not sure what the error message means, or where to look to debug it.

Thanks,
Joe

Your upstream, the server that nginx is proxying for closed the
connection before sending a response back to nginx.

I’ll take a guess that this is a rails app, and the upstream server is
mongrel, turn on the mongrel debug log (kill -USR1 ?) and see if you
can replicate the problem.

Cheers

Dave