The other day, I attempted to use my Snapshot Test World, but it wasn't showing up in the world list. Previous snapshots which could run it also weren't showing it, even 1.5.2. I thought the world was corrupted, but I did a system restore to the last time the world worked and it still wouldn't show.
I renamed my .minecraft folder and let the launcher reinstall it. I copied the world into the new saves folder and it works again. However, I'd rather not do a full reset of my folder just to fix this. Any ideas what could cause the game to refuse to load a single world independent of the save folder or jar/version setup? What setting file or whatever?