Day 4: 2:00 P.M.-3:00 P.M. - S4-E8
Factual error: When Edgar proves that Mary-Ann Taylor framed Sarah to be the rat in CTU, he claims that she remotely took control of her computer by sending the signal over "a plain old AC power line." This is not possible, unless the computers were equipped with special hardware to separate the AC from the signal. If they were, this traffic would have been observed by James Heller's assistant.
Day 4: 9:00 P.M.-10:00 P.M. - S4-E15
Factual error: The bad guy uses the thumb to access the Air Force base at the end of the episode. Thumb scanners are activated by the pulse and wouldn't work otherwise.
Day 4: 12:00 A.M.-1:00 A.M. - S4-E18
Factual error: In the opening scene of the episode, Marwan gives a cohort Lat/Long coordinates for a nuclear missile. The computer shows these coordinates as being in the Midwest, they are actually in Nevada. (37 14' N 115 21' W).
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.