Continuity mistake: Agent Wells is chasing the bad guy on foot. In the first shot her SIG is not cocked. In the next shot it is cocked. In the next shot the slide is retracted. In the next shot she somehow magically fires it. Then the hammer is somehow forward again. A few seconds later as she climbs the stairs it supernaturally transforms into a Glock. And at no time does she use a proper grip on the gun.
Continuity mistake: When Agent Wells drives the van filled with bombs out of the parking structure both back doors of the van are open revealing the bombs, in the next shot both doors are closed.
Continuity mistake: Agent Wells spots Lozano leaving the Pentagon and begins a high speed chase. During that chase Lozano is alternately wearing and not wearing his seat belt. In addition, Lozano's car is only damaged in the close-up scenes; whenever the car is shown from a distance, it is completely undamaged, even after Wells has rammed him repeatedly.
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.