Hobbes dishes out a 502 error that seems to be this catalyst error:

"2011-02-23 23:09:13.560377500 [error] Caught exception in engine "store_session: data too large"

The nginx access log line is:

10.1.1.14 - - [23/Feb/2011:23:09:13 +0000] "GET /release/c51cddba-7ba0-3e88-a933-159fd9de9994/edit HTTP/1.0" 502 173 "http://test.musicbrainz.org/release/c51cddba-7ba0-3e88-a933-159fd9de9994" "Opera/9.80 (Macintosh; Intel Mac OS X; U; en) Presto/2.2.15 Version/10.10"

and the nginx error log says:

2011/02/23 23:09:13 [error] 27215#0: *64907 upstream closed prematurely FastCGI stdout while reading response header from upstream, client: 10.1.1.14, server: test.musicbrainz.org, request: "GET /release/c51cddba-7ba0-3e88-a933-159fd9de9994/edit HTTP/1.0", upstream: "fastcgi://127.0.0.1:55901", host: "test.musicbrainz.org", referrer: "http://test.musicbrainz.org/release/c51cddba-7ba0-3e88-a933-159fd9de9994"

This is a transient error that does not always occur though nikki can reproduce it most of the time.