No idea, really. But I do know I did some sloppy "do before I think" ad
hoc SQL work when I created the table that has the corruption, and I
note that IBSurgeon writes in one of their docs that dropping tables
while other transactions might be working (trying to work?) with it, can
confuse the engine and cause corruption. If that is the case (which I
find a bit scary) it might be what happened to me (although I was alone
working with a couple of transactions fiddeling with that table back and

If you create many pseuvdo-temporary tables like TMP646474 and then drop them, while users are active, you are on the way to corruption.
The simple workaround is to pump all data to the empty database with the same structure, except bad table.
It could take several days, but it's free.

Regards,
Alexey Kovyazin
www.ibsurgeon.com