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)
Day 3: 11:00 A.M.-12:00 P.M. - S3-E23
Revealing mistake: When Sherry is shot, the impact is visible on her shirt, but no gunshot wounds can be seen on her skin underneath, which can be seen as she falls to the ground.
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.