Visible crew/equipment: When David is talking to his daughter, just after the remark about there being nothing to worry about, a camera moves into shot on the right hand side of the screen. (00:08:50)
Revealing mistake: When Dan is shot at the end, you can see protective equipment under his shirt as he falls back. (00:40:40)
Continuity mistake: When Alan tells Teri he will stay with her until they find Kim, Teri's bag strap falls off her shoulder. Camera changes and the strap is back up on her shoulder. (00:32:30)
Continuity mistake: When Teri is talking to Alan about Kim failing Algebra, she has her hand on her bag in one shot and on her coffee in the next. (00:31:40)
Answer: According to Jack in season 5, Nina was "deep cover" before the pair ever met, so it's safe to assume she was recruited before she joined CTU. As for what her primary mission was, who knows? Perhaps it was to gather sensitive intel and pass it on to her superiors. Given that she worked for an intelligence agency, it's not a stretch. Or perhaps we may find out in season seven.
Cubs Fan