ERROR: wtw-intl.epub/OEBPS/toc.ncx(110): ‘heading_id_17′: fragment identifier is not defined in ‘OEBPS/Text/wtw-intl.html’

I got this error (actually, like 200 of them) from the EPUB validator that ThreePress very generously hosts, and also from iTunes Producer when trying to upload a file for the iBookStore.

I tried FlightCrew too, another tool I’m super happy to have, but got a similarly unhelpful error message:
file: wtw-intl.epub/OEBPS/toc.ncx
line: 110
message: This element’s “src” attribute value is “Text/wtw-intl.html#heading_id_17″, but an element with an ID the fragment is referring to does not exist in that file.

OK, that’s English, but after double-checking the files and values it mentioned, I still couldn’t identify the problem.

After banging my head against the wall for 30 minutes, I decided to run Sigil‘s check and discovered a few seemingly unrelated errors. So I closed a couple paragraph tags that were left open (ah, the perils of hand-coding), and corrected a couple of typos in a couple of internal anchored references. I then re-ran the file through EPUB Check and, presto change-o, errors gone. Nothing to do with the table of contents file (toc.ncx), and nothing to do with that particular id value. Yeah, that totally makes sense

I’ve previously mentioned the epubcheck wiki and the mobileread epub forum as helpful sources of info for error troubleshooting, but I wonder if there are others.

Can anyone recommend any other great diagnostic tools or resources for EPUB errors?