I recently turned on debug mode and notices and saw a few errors about missing inactive_*_tab indices. They look like they are harmless, but I decided to fix them anyway since the fix was simple and got rid of some "noise" in the debug output.
It looks like there are 3 locations where these values are used as indices into an array but the entries are never created in any array anywhere (that I could find by searching). They may be obsolete entries for a feature which is now gone and maybe these lines can be deleted, but I decided to just put tests around them to only make the string substitutions if they are set in the array. This made the notices go away and the code is still there in case some other themes may use this feature.
In include/themes.inc.php, find these 2 lines in 3 places (NOTE that the text <some LINK substitutions> varies by location):
$theme_alb_list_tab_tmpl['inactive_next_tab'] = strtr($theme_alb_list_tab_tmpl['inactive_next_tab'], <some LINK substitutions>);
$theme_alb_list_tab_tmpl['inactive_prev_tab'] = strtr($theme_alb_list_tab_tmpl['inactive_prev_tab'], <some LINK substitutions>);
and put the following tests around the lines (preserve the original lines above where you see <Same text as the line above> here):
if (isset($theme_thumb_tab_tmpl['inactive_next_tab'])) {
<Same text as first line above>
}
if (isset($theme_thumb_tab_tmpl['inactive_prev_tab'])) {
<Same text as second line above>
}
Some of the themes may have similar code (I also found these lines in the sample theme) and so may need the same tests inserted in them, especially if you based a theme off of the sample theme.