Day 3: 8:00 P.M.-9:00 P.M. - S3-E8
Revealing mistake: Right when Jack and the prisoner are walking through what appears to be the gun room in the prison, you can see the guy who attacks them through the door. You can only see a small part of him but he is obviously waiting to enter the scene and attack. (00:14:00)
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.