In Drupal 6, one can go to the Site Configuration -> Site Information page, and at the bottom there is a setting titled "Default Front Page". The site URL (in text) there is unchangeable via the interface. However, we have the wrong value there.
This came about because I used the Backup/Restore module to copy a production site's data to a test server. It has to be a data problem because that is the single change that's happened. However, I'm completely at a loss how to fix it. The original site's URL is all over the database.
Simply wiping the database and reinstalling isn't an option, because we need the node, user, CCK, etc data from that production server.
There are two suggestions I've seen all over the web to fix this: change the settings.php value, and empty all Drupal's cache_* tables.
In our case, settings.php hasn't changed, and is correct. And we have both used the Admin Menu interface to empty the cache, and have manually gone in and made sure all cache tables are empty.