Day 4: 8:00 A.M.-9:00 A.M. - S4-E2
Continuity mistake: When Agent Lobell tries to stop a car from leaving the train station, the terrorist in the car shoots at him through a window, shattering it. When the camera cuts to a shot of Lobell falling to the ground, the window in the car is gone, but no fragments of it are on the car or the ground under it.
Day 4: 6:00 P.M.-7:00 P.M. - S4-E12
Continuity mistake: When Driscoll leaves CTU, Secretary Heller asks Tony to call Division to request a replacement. Moments later, Heller says he still wants to call District. (00:14:00 - 00:14:40)
Day 4: 3:00 P.M.-4:00 P.M. - S4-E9
Continuity mistake: In the previous episode, when Jack and Tony try to stop the helicopter, they drive off the road and hit the front bumper and lose their front license plate. This is also visible in this episode as Jack and Tony wait for backup and when they drive off there is still no license plate, but about 8 minutes later, when they arrive, the license plate is back on. Since the show is in real time and they arrived in the exact time they said they would, they couldn't have stopped to get it fixed or anything.
Day 4: 7:00 P.M.-8:00 P.M. - S4-E13
Character mistake: Near the start of the episode, Curtis asks how many square miles are affected by the blackout, and is told "approximately 8." A few minutes later, Sarah states that the blackout area is "everything within an 8 mile radius." These are vastly different amounts. An 8 mile radius would cover approximately 201 square miles. (00:02:25 - 00:06:55)
Day 4: 1:00 A.M.-2:00 A.M. - S4-E19
Continuity mistake: At the very end of Episode 18, Jack knocks out Joe Prado after breaking his thumb and getting information from him. At the very beginning of Episode 19, we see Prado already being treated for his injuries, with a hand splint, and nurses checking his head, within the CTU clinic. Even if you factor in the opening recap sequence at the beginning of every episode, that means that Joe Prado regained consciousness, Jack unhooked his handcuffs, and dragged him back to CTU in three minutes or less.
Day 4: 10:00 P.M.-11:00 P.M. - S4-E16
Continuity mistake: When Jack shoots Nicole through the wall, she slides down, smearing blood on the wall. However when Jack comes to check if she is dead, no blood is seen on the wall.
Day 4: 12:00 P.M.-1:00 P.M. - S4-E6
Revealing mistake: During the rescue, a few shots after SecDef Heller announces to Jack that he's also out of ammo, you see a shot of the bullet-riddled car taking a few more bullets. The new bullet holes obviously fade in.
Day 4: 2:00 P.M.-3:00 P.M. - S4-E8
Continuity mistake: When Curtis Manning is chasing Mary-Ann Taylor out of CTU, you see Mary-Ann exiting through a solid grey door marked by an EXIT sign in a wall which looks like concrete. As she passes through the door, you can see sunlight on her from almost directly above. In the next shot Mary-Ann is seen exiting through a glass door in a wall of glass panels. Unless there is a section of CTU's roof missing, the room she exits from in the second shot is not the room she exits to a few seconds earlier in the first shot.
Day 4: 8:00 P.M.-9:00 P.M. - S4-E14
Character mistake: In the scene where Chloe uses traffic video to get the license plate number off the van that takes Jack and Dina Araz to Habib Marwan, Tony is seen calling the field teams with the plate number, saying that it's "Charlie-Victor Alpha-Three-Two-Two." But the plate reads "6CVA322" so he should've said, "Six-Charlie-Victor-Alpha-Three-Two-Two."
Chosen answer: Nobody knew, not even the writers, until at least halfway through the season when they were writing the end. They wanted to keep all options open.
Nick N.