> We have experienced out first database corruption from any of our customers
> in 15 years of Interbase and Firebird!
>
> I would like some clues as to what may have caused it.
>
>
>
> FB 1.5.3, windows server 2003, superserver

Read ReleaseNotes for 1.5.5 (and, better for 2.1.2) and seek how many
index related error was fixed. Then ask yourself - why i still use 1.5.3 ???

Without run of gfix -v -f it is impossible to said even which kind of corruption
you encountered. Upgrade at least up to 1.5.5 ASAP and consider upgrade to
2.1...

Regards,
Vlad