Visible crew/equipment: Beth attempts to convince Norman to inject himself with a tranquilizer by trapping him in a room and filling it with water. As the water rises, Norman frantically looks for a way out. There is a quick overhead shot of this scene that flashes on for just a second, and if you look around the perimeter of the room - you will see the entire crew, complete with camera and lights.
Revealing mistake: As Ted is being burnt by the fire, you can see his body trapped underneath the debris. In a side shot of Ted it is very obviously a stunt dummy as the left arm and head stay completely still and the arm remains completely stiff despite hearing screams and shouts from the pain.
Continuity mistake: When Norman and Harry are talking in the dorm after they hear of the storm coming, Harry is lying down in the bunk telling Norman how they are going to die. There is a shot of Harry's leg at the end of the bunk, then Harry sits up against the wall and in the next shot his leg is still at the end of the bunk, but wouldn't it have moved back when he sat up against the wall?
Other mistake: They're meant to be on rebreathers (they recycle the air around the system, so you 'rebreathe' it), yet in the u/w scenes, it can be seen that they're on open circuit systems, i.e. conventional scuba, as they're venting exhaust gases. Rebreathers do not release bubbles in the quantities shown.