Continuity mistake: When Four uses the wristband to free himself from the cell in Erudite as Tris and Christina engage the guards, as soon as the door opens he drops the wristband. In the next shot when all the guards are down, the wristband has changed positions on the floor.
Suggested correction: It's in the same position, it's just the camera angle has changed. Look at in the first shot you can see 3 windows and the light from those 3 windows, which when the camera moves, we only see light from 2 windows. In the next shot, we only see 1 window and just a little bit of light from the 1 window.