When a headline begins with a number this is put as id to the headline element as it is. Because XHTML does not allow the id of an elemente to start with a number this leads to a XHTML error/warning. I think this behaviour should be changed. For example by generaly prepending something to the id or onlay to headings with numbers.
BTW: What happens if two headings have the same name? This would also lead to a XHTML Error.