Error Message Feedback

Hi Gentlemen,
can you explain?
Quite often Markup Validation Service says something like:
"Sorry, I am unable to validate this document because on line 479, 524  
it contained one or more bytes that I cannot interpret as us-ascii  (in 
other words, the bytes found are not valid values in the specified 
Character Encoding). Please check both the content of the file and the 
character encoding indication."
But my pages don't contain any visible mistakes and that Linux Tidy says 
that everything is OK. But when I apply CSE HTML Validator Pro v7.0 tidy 
option, your service validates my HTML code immediately.
I'd like to know what the bytes that "can't be interpret as us-ascii" 
are meant and how to fix this problem.
Alex.
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