Visible crew/equipment: When Jack, Kim and Teri are at the compound, Kim stands up shortly before Gaines' first shot. On the left you see a crew member kneeling. (00:26:20)
Continuity mistake: When the CTU units arrive with the helicopter, they ask Teri to drop the weapon. She puts down the weapon, than it is a cut to a view from behind Teri and she still has the weapon in her hand, but is starting bend to put the weapon down.
Continuity mistake: In the scene where the CTU tactical team arrives at the 'lz', the chopper that first makes ground contact is marked "CTU-1", and offloads six federal agents in full tactical gear. Later in another shot of the same scene, you see "CTU-1" marked on a second chopper still circling in the air. It's clear this is not the same chopper as before, as we see the first "CTU-1" on the ground again moments later.
Audio problem: When Jack and Rick are talking in the water tower, a lot of the audio in that scene is either out of alignment or overdubbed. Easiest to spot is when Jack is checking Rick's wound. (00:10:30)
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.