This looks to be fixed. SMF got itself all confused somehow and hosed part of the database. Some db repair seemed to clean things up, and I upgraded SMF 2.0.9->2.0.10 since it included some bug fixes, though nothing that appeared to matter for us, maybe something there.
A small number of posts that came through today ended up getting deleted. That Russian thread Jared linked became the home of all new threads somehow for a period of time, and they ended up duplicated multiple times. I split out most of what ended up in there into their own threads. That thread had gotten up to 15+ pages of mostly duplicated posts so I'm sure I missed something. That thread itself also was deleted as it was otherwise weird.
All forum totals and counts were re-calculated after cleaning that up, as something to do with the weird thread messed up some people's counts.
Everything checks out fine now in SMF's db checks, and none of the errors that were flooding its error log have recurred since coming out of maintenance mode, so pretty sure this is all fine now.