When using http://musicbrainz.org/ws/1/user with username that contains the exclamation mark, no matter what password you enter, you get a HTTP 400 error.

$ curl --digest --user 'test!:test' -v 'http://musicbrainz.org/ws/1/user?type=xml&name=test%21'

• About to connect() to musicbrainz.org port 80 (#0)
• Trying 72.29.166.157... connected
• Connected to musicbrainz.org (72.29.166.157) port 80 (#0)
• Server auth using Digest with user 'test!'
  > GET /ws/1/user?type=xml&name=test%21 HTTP/1.1
  > User-Agent: curl/7.21.3 (i686-pc-linux-gnu) libcurl/7.21.3 OpenSSL/0.9.8o zlib/1.2.3.4 libidn/1.18
  > Host: musicbrainz.org
  > Accept: /
  >
  < HTTP/1.1 401
  < Date: Tue, 13 Dec 2011 14:37:59 GMT
  < Content-Type: text/plain; charset=UTF-8
  < Connection: keep-alive
  < Keep-Alive: timeout=20
  < Server: nginx/0.7.65
  < WWW-Authenticate: Digest realm="musicbrainz.org", qop="auth,auth-int", nonce="0I9vlagoSES0sssMJJy4dQ==", opaque="C/vJ9jIQQQSy0t1QHkgsxA==", algorithm="MD5"
  < Content-Length: 23
  < Access-Control-Allow-Origin: *
  <
• Ignoring the response-body
• Connection #0 to host musicbrainz.org left intact
• Issue another request to this URL: 'http://musicbrainz.org/ws/1/user?type=xml&name=test%21'
• Re-using existing connection! (#0) with host musicbrainz.org
• Connected to musicbrainz.org (72.29.166.157) port 80 (#0)
• Server auth using Digest with user 'test!'
  > GET /ws/1/user?type=xml&name=test%21 HTTP/1.1
  > Authorization: Digest username="test!", realm="musicbrainz.org", nonce="0I9vlagoSES0sssMJJy4dQ==", uri="/ws/1/user?type=xml&name=test%21", cnonce="MDIyMDAx", nc=00000001, qop="auth", response="cbbd17c36b5157cc857fdc1eaa9318ad", opaque="C/vJ9jIQQQSy0t1QHkgsxA==", algorithm="MD5"
  > User-Agent: curl/7.21.3 (i686-pc-linux-gnu) libcurl/7.21.3 OpenSSL/0.9.8o zlib/1.2.3.4 libidn/1.18
  > Host: musicbrainz.org
  > Accept: /
  >
  < HTTP/1.1 400
  < Date: Tue, 13 Dec 2011 14:37:59 GMT
  < Transfer-Encoding: chunked
  < Connection: keep-alive
  < Keep-Alive: timeout=20
  < Server: nginx/0.7.65
  < Access-Control-Allow-Origin: *
  <
• Connection #0 to host musicbrainz.org left intact
• Closing connection #0