Factual error: The Pakistan Air Force officer in the airport is listed in the credits as 'PAF Lieutenant' and is wearing the two pips of an army lieutenant. In fact, the PAF still uses RAF-style ranks, so he should be a Flying Officer, and the PAF also uses distinctive eight-pointed stars instead of pips.
Other mistake: When Carrie leaves through the tunnel to the street, when she comes out of the store there's a shot of 2 people kinda looking at her. That shot was horizontally flipped - look at the sign on the door. (00:34:05)
Continuity mistake: When Lt Col Khan leans forward in the airport scene his epaulette can be seen to bear only the single pip of a second lieutenant. When he stands up again he is wearing the pip and emblem of a lieutenant-colonel once more.
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.