This is often caused by a leftover end tag from an element that was removed during editing, or by an implicitly closed element (if you have an error related to an element being used where it is not allowed, this is almost certainly the case).In the latter case this error will disappear as soon as you fix the original problem.For instance, using XHTML's "self-closing" tags for "meta" and "link" in the "head" section of a HTML document may cause the parser to infer the end of the "head" section and the beginning of the "body" section (where "link" and "meta" are not allowed; hence the reported error). This might mean that you need a containing element, or possibly that you've forgotten to close a previous element.
The validator has found one of these undefined characters in your document.
The character may appear on your browser as a curly quote, or a trademark symbol, or some other fancy glyph; on a different computer, however, it will likely appear as a completely different character, or nothing at all.
Your best bet is to replace the character with the nearest equivalent ASCII character, or to use an appropriate character entity.
For more information on Character Encoding on the web, see Alan Flavell's excellent HTML Character Set Issues reference.
This error can also be triggered by formatting characters embedded in documents by some word processors.