Your wandering through the Nether and you see a Ghast in the distance, she notices you as well and starts to get scared

A-ah?! W-w-who are you!!! I've never seen you here b-b-before!!! >~<

You need to calm this scaredy bean down or she'll shoot a fireball at you, she's not trying to though, it's just a habit to do so at this point for her. :]