Continuity mistake: When Saul is at the terrorists' house talking to a neighbour, his arms are either falling down straight or bent, depending on if it is a front or a back shot.
Continuity mistake: SPOILER - When Brody and Carrie are in the car having sex with her laying down, the shots behind her head show a big amount of space in-between her head and the door, yet the front shots show it squashed against the door.
Continuity mistake: When Carrie and Brody meet at the bar, his hand's position is inconsistent between shots.
Chosen answer: Everyone else calls him Brody or Sgt. Brody, and it probably became habit to her.
The_Iceman
Can't get the source but I read that even the Producer responded to that. Stating that it was common practise for a military wife living in a military complex as everybody refers to their husband by their last name.