Other mistake: During the submarine interior scene, there are several dead seamen shown floating about. One of them is pretty close to the camera and floating with his eyes open. He blinks before the shot ends.
NickiOther mistake: When the water tentacle enters the rig, in one shot in the 4:3 version of the film on video you can see that the tentacle is a superimposed special effect because of the dead straight line where it is supposed to have come out of the water.
Other mistake: When Ed Harris is dropping down the Abyss there are a couple of time that bubbles come out of his mouth or can be seen in his nostrils, even though he is supposed to have been breathing liquid for quite some time. (02:13:28)
Answer: The crew are working at that pressure and only need diving suits and submarines for long periods in the ocean and to breathe longer. Hence when they return to the surface in normal conditions, a slow return to the surface is required to avoid their bodies developing a condition called the bends. This did not happen at the end as they were protected by the aliens and their ship.