Zend_Db_Adapter_Pdo_Mssql::_dsn() is including $this->_config['options'] in the DSN. Example:

$db = new Zend_Config(array(
    'adapter'  => 'PDO_Mssql',
    'params'   => array(
        'host'     => 'localhost',
        'port'     => '1433',
        'dbname'   => 'db',
        'username' => 'user',
        'password' => 'pass'
    )
));

$db = Zend_Db::factory($db);

Results in the following DSN being generated when Zend_Db_Adapter_Pdo_Mssql::_dsn() is called: dblib:host=localhost:1433;dbname=db;options=Array

Fix is addition of the following to along with the other existing unsets() in 'Zend/Db/Adapter/Pdo/Mssql.php':

unset($dsn['username']);
unset($dsn['password']);
unset($dsn['driver_options']);
unset($dsn['options']); // Add me

This bug does not affect functionality since the addition of "options=Array" in the DSN is ignored, but I believe it is invalid none the less.