It could happen.

I've never been one to believe in aliens from outer space. I mean it's absurd to think that some extra terrestrial life force could land on Earth and wreak havoc. Wouldn't we be able to recognize it right away? For starters it wouldn't have human feelings so it would probably give itself away by acts of cold inhumanity or sheer destructiveness. And even if it didn't, it wouldn't look quite like us, would it? I mean it'd probably have some kind of creepy features, too tall or lifeless eyes or no hair or far too many teeth, something like that. Not that it would ever happen, but if it did—and that kind of creature ever showed up—I would hope that the people of Earth would banish it as soon as possible before it prepared the path for even more such horrors.