Mage_Cms_Block_Widget_Interface
Heads up! This article was written over 15 years ago. While it may still be helpful, please verify any information, as my perspectives and practices may have evolved.
For anyone following the old widget tutorial in Magento Knowledge Base you’ll find yourself getting the error:
Fatal error: Interface 'Mage_Cms_Block_Widget_Interface' not found
This is because the interface has been renamed Mage_Widget_Block_Interface in at least version 1.4.0.1.