When can uid 1 get an access denied page? I am having a very strange problem that I can't track down.

We have been working on a custom site for a client now for several months.  Theme is a Zen subtheme.

I can't remember the last time I tried to access it, but I am getting an "access denied" when I try to view /admin/build/themes/settings/mysubtheme, even as uid 1.  It is getting logged this way, too, so I think it really is an access denied.  I can access this page for Zen, Rootcandy, and other enabled themes.

I have done all of the usual tricks for attempting to fix this (run cron, clear cache, rebuild node perms, assign all perms to my superuser role, etc) without luck.

A theme-settings.php exists for the theme and all of the hooks are named properly for the theme.

How is this possible since user_access returns TRUE when uid==1?

Thanks.
After trying to fix this for an hour, I tumbled on the solution right after I posted this.  Apparently, it is possible to set a theme as "default" but not have it "enabled", and have a site function normally.  Enabling the theme fixes the problem.