If you receive this message, it means that the feed headers are being sent as one encoding, but the feed itself is likely declaring another one. This can be problematic for the contents of the feed.
Generally within an XML document you will see the first line read something like this:
<?xml version="1.0" encoding="utf-8"?>
This means that the feed is being declared as UTF-8. But if the headers of your feed don’t match, then the contents might be scrambled when received on the other end. Luckily, this can be a simple fix.
In order to make the header match the feed declaration, you should check your .htaccess file (this may not be the case if you are using something like IIS or Lighttpd, but is common if you use Apache). In the .htaccess file, you should have a line like this:
AddCharset utf-8 .xml
This line associates the encoding (utf-8 in this case) with a particular extension (.xml). And that’s all there is to it. Now the header, sent prior to the file, will match the encoding specified in the document.