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