Well, in this case it was real bad.
Here’s a tip: When using the “RSS widget” in WordPress, do not point it to any web-based feed. While moving files around, I managed to make the Gallery RSS feed completely nonexistant, which meant that WordPress showed its 404 page. WordPress then tried to fetch the un-fetched Gallery RSS feed to show it in the sidebar on the 404 page. The RSS feed wasn’t there, so WordPress returned the 404 page…
The Gallery RSS feed is fixed, and the WordPress 404 page no longer shows sidebars.
In case anybody’s wondering why my blog showed a generic holding page for the last 12 hours….