Zend_Db_Table_Abstract is not friendly with Zend_Loader_Autoloader_Resource when defining a custom Row Class. The Zend_Loader::loadClass($this->_rowClass); tries to load the Row Class and fails/throws exception.

class DbTable_Test extends Zend_Db_Table
{
   protected $_rowClass = 'Row_Test';
}

Workaround:

// Let the Zend_Loader_Autoloader_Resource load it prior to DbTable usage
new Row_User();

// DbTable stuff
$db = new DbTable_Test();
$select = $db->select()->where('id = ?', 1);
$row = $db->fetchRow($select);

// Row_Test should have loaded correctly / not thrown exception
Zend_Debug::dump($row);