Default theme goes missing?

Anyone has issues with Drupal 7 "losing" track of your current theme? My custom theme is still active, but not enabled or set as default anymore in Appearance. In the logs, I get errors of undefined index in theme_get_setting. I'm not sure if there's any other problems relating to this happening, other than it seems weird to me.

I'm using a custom subtheme, with Adaptive Theme as my base theme. I do this in Drupal 6 all the time, and have never seen this happen before.