Registration in 0.64 doesn't only add the info users input when registering into the table $prefix_users. It also adds the name of the default theme, and quite possibly the default number of stories on the main page.
So: never change the name of your default theme. If you want a new theme as default, delete the old theme and name the new theme the old name (use the find/replace function to change the name of all the links so they go in the right directories). And don't take changing your default number of stories lightly.
If you have already changed your default theme, you can remedy this by putting copies of this same theme in your themes directory, with the different names (again, remember to change those links). Or if you just delete the old themes, users get the default theme--I didn't like this option since I wondered about possible bugginess or loss in processor time. Or you can make a dump of your db table users and edit everybody's theme to default. No, edit users doesn't let you mess with your users' themes. It should, as a means available for punishing naughty users--give 'em a really irritating theme and make them change back. Or give 'em one with a parse error, and as long as they don't reset their cookies, they are denied access!! Btw, you can always do this if you just edit your table _users and throw in that buggy theme.
A suggestion for futher PN versions: theme and number is not submitted to the db at registration, and only when users explicitly change themes--otherwise it's default.
The core still needs some work. There's this, there's search's requiring the admin's name in the db to return results. There's the hyperbolic stats module. A ton of applause to the dev team for all the sweat they've already put into PN. Let's cool off when it comes to asking for new features and let them get the core perfected.
This little story dedicated to Talen who helped me find this glitch--btw, Talen, welcome to postnuke.