If you search through recent 9.2 or 9.3dev PDFs for the string "??"
you should find ten occurrences. Other than the first instance in
Chapter 9, the other nine are all in the index, and indicate an
unknown reference to some index entry. For example, you can see in the
current 9.2-A4 PDF:
"""
deadlock, 349
timeout during, ??
...
fsync configuration parameter, ??
...
full_page_writes configuration parameter,
??
"""
AFAICT tex is getting confused by <indexterm> entries appearing before
<term>. Or at least, switching the order seems to fix the problem.
Patch attached.
Josh