Factual error: At the very beginning of the episode, during Agent Wells' escape in the blue van, there is a very tall skyscraper in one shot. There aren't large skyscrapers like that in DC. Also, when she crashes the van into the Tidal Basin, where did all of the cherry trees that surround it disappear to?
Continuity mistake: When the FBI agent is chasing down "Catalan", after crashing his car, suddenly in one shot the slide of her pistol is retracted, as if the gun is empty. Previous shots it is closed and ready to fire. Then right after showing it retracted and locked back, the very next clip she fires her gun, without doing anything about the slide.
Continuity mistake: When Hannah reads the email about The Traitor she has blood on her face, when she arrests Whitaker she is clean and when she remembers the past she has blood on her face again.
Answer: It's a recurring theme on the show. Characters come and go depending on the storyline. General Cochrane, Tyler Richmond, Abe Leonard, etc.