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?