As you can see only good result is saved. If autentication is wrong, then result is ignored and it's not stored. But return is false. That's right.

public function authenticate(Zend_Auth_Adapter_Interface $adapter)
    {
        $result = $adapter->authenticate();

        if ($result->isValid()) {
            $this->getStorage()->write($result->getIdentity());
        }

        return $result;
    }

Real issue rise up with redirection Zend_Auth::getIdentity(). It returns succes result.

 public function getIdentity()
    {
        $storage = $this->getStorage();

        if ($storage->isEmpty()) {
            return null;
        }

        return $storage->read();
    }

So there're two ways how to get result of autenticate but unfortunately it gives another result:

$this->_auth->authenticate(new My_Auth_Adapter_User("ondra@mojestopa.cz","******")); // success
$result = $this->_auth->authenticate(new My_Auth_Adapter_User("ondra@mojestopa.cz","wrong password")); //failure

echo $this->_auth->getIdentity()."
";  // print "ondra@mojestopa.cz"
echo $result->getIdentity()."
"; // print ""

I hope you understand, what i'm talking about. I think it should gives defaultly failure in both ways.