I've not looked at the relevant code, but I think there may be a small bug in the topic splitting? After splitting a topic, "Find and repair any errors" says that the subject of the newly split topic is "currently not stored in the subject cache". Note I don't have caching enabled in the "Server Settings" options, if that makes any difference.
There's a whole bunch of things that will cause that error. It will apparently be fixed in the next SMF 1.1 release.
If you go the Forum Maintenance section of the Admin Center and empty the unimportant logs, then go to Find and fix errors, it will moan that ALL the subjects are missing from the cache. :P
- Rpg Cyco