In case no character encoding info can be found through the usual algorithms, the validator outputs a 
warning about that missing info, e.g:
http://validator.w3.org/check?uri=http%3A%2F%2Fqa-dev.w3.org%2Fwmvs%2FHEAD%2Fdev%2Ftests%
2Fnocharset.html&charset=%28detect+automatically%29

However, if the document has bytes that cannot be mapped as utf-8 (or whatever fallback charset), the 
fatal error message (fatal_byte_error) alone is given, without the usual warning about lack of encoding, 
which can be confusing, e.g:

http://validator.w3.org/check?uri=http://qa-dev.w3.org/wmvs/HEAD/dev/tests/
nocharset_notutf8.html;ss