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http://qa-dev.w3.org/wmvs/HEAD/check?uri=http%3A%2F%2Fwww.w3.org%2Ffoo properly says "404 not found", but http://qa-dev.w3.org/wmvs/HEAD/check?uri=http%3A%2F%2Fwww.w3.org%2Ffoo;output=xml reports one error "<msg line="1" col="0" offset="0">end of document in prolog</msg>"
earl and n3 output modes show similar behavior.
Partly fixed in CVS - the X-W3C-Status header does not state "Invalid" anymore.
It appears to still be broken, even the HTTP headers part, for 4XX status codes.