When variable is typed as interface and we use concrete implementation in code, sometimes there are valid calls to methods that doesn't exist in interface. For example when using Symfony\Component\HttpFoundation\Session\SessionInterface and want to use getFlashBag method, this will trigger error in PHPStan (level 2):

/** @var \Symfony\Component\HttpFoundation\Session\SessionInterface $session */
$session = $request->getSession();
if ($session && method_exists($session, 'getFlashBag')) {
    $session->getFlashBag()->add('success', 'False positive detected!');
}