Why was Kisame surprised when he saw Obito (Tobi/fake Madara) without the mask the second time they've met in person?
I dont believe this is a plothole, because the first time Kisame met the "Madara", it was actually Obito disguised as Madara Uchiha, Although we never got to see the actual person that Kisame was talking to, based of the way the persons hair from the Shadow was, it looked exactly like Madara Uchihas hair, and we all know that Obito's hair looks very different and isn't spiky long. (I would guess that Obito probably used substitution Jutsu in order to resemble the real Madara.)

However, the second time, he saw Obitos real face and that probably caught him by surprise since he was not expecting those scars.