I recently installed Mike Little's theme Journalized 2.0 (http://zed1.com/journalized/themes/journalized-theme-version-2/) on my blog, with some modifications. The theme is supposedly compatible with WP 1.5.2, which is what I'm using, and at first, it appeared to be working fine.
However, overnight, the theme deactivated itself and activated the default Kubrick theme!! I have no idea what would cause this, but needless to say, it is very disconcerting. It actually happened on two separate occasions. I then re-activated Journalized and moved all of my other themes, including the "default," to a different folder, in hopes that WP would be unable to spontaneously switch themes if no other themes existed. But an hour or so later, it did indeed attempt to switch to the "default" theme -- and since no default theme existed, my blog went totally blank. (I ascertained what had happened by looking at the options table in my MySQL database and seeing that the template had switched to the word "default," again without any input by yours truly.)
It's like my WordPress installation is having an allergic reaction to Journalized! After an hour or two, it automatically deactivates it and attempts to switch to the "default" theme. I realize this isn't a Journalized forum, and I did leave a comment on Mr. Little's blog... but does anyone have any idea what could cause something like this?