Revealing mistake: When the boat is supposedly surrounded by a swarm of flies, they're really just relatively few flies trapped between two panes of glass in front of the camera lens. Evidence: whenever the camera moves, the "swarm" moves with it against the background, and you can even see some of the insects walking on the surface of the glass.
Revealing mistake: Charlie calls Rose to take the tiller, for the first time. She has a hamper on her lap, but there's nowhere to put it, so she takes it out and lowers it down below the bottom of the screen, then she takes her hands away and the hamper quickly disappears. Some crew member must have taken it away.