We try to notify this list when API changes are made in Drupal 7, so contrib and custom developers have a chance to fix their code before they have the pain of figuring out why it's broken.

FIrst, system_settings_form() had until this weekend an "automatic defaults" capability, removing the need to set '#default_value' => variable_get('whatever', 'def') in your system settings form. The automatic defaults capability was rolled back (issue) so that means that wherever you have been using system_settings_form() without explicitly setting the #default_value to a variable_get(), you will have to go back in and set it up as you would have in Drupal 6. This is a significant API change and will have widespread impact, so check your system settings forms.

Second, hook_menu_active_handler_alter() was removed from Drupal 7 (issue), so if you have used that, you will have to rework your code.

Thanks,
-Randy

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Randy Fay
Drupal Module and Site Development
randy@randyfay.com
+1  970.462.7450