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

[Joe Van Dyk]

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