TL pointed out that when an editorial note appears in a field such as an author field, and the author field is used in a TOC table where the rest of the document itself is not available, the note number link was attempting to pop up the note but showing a not-found message because of course the note is not available in the TOC page.

I've worked around this by catching situations like this in the XSLT; when such a note appears in content which is inside an XHTML table cell (this is the structure produced by the mdtList.xql TOC-generator), the note is processed differently, and does not turn into a link. The note content is still accessible with a mouseover because its content is in the title attribute anyway. An extra class changes the cursor to "help" instead of "pointer".