When the divers exit the Seaview hatch to find the missing part, they're using dual air tanks with double-hose regulators. When they return, they're wearing single tanks with single-hose regulators.
The lower budget of television can often be spotted on "Voyage". The production department used some stock footage of a runway zipping by without any consideration for reality. When the flying sub is approaching National Airport all of the views outside of the cockpit window show the landscape moving from left to right, and then right to left, suggesting that the craft is somehow flying sideways, and in two different directions.
There are two goofs when Weber is holding the crew at bay with his pistol. The pistol is in the slide lock position, indicating that the gun is out of ammunition. Experienced crewman on the Seaview would have known that his gun was useless. Later, when they do attack Weber, the slide mechanism is suddenly in the correct forward position, showing the gun as it should have been all along.
When the Seaview is being hit by falling ice chunks, interior shots looking out the nose windows show the usual 4 window arrangement, but external shots show an 8 window configuration. The exterior shots are stock footage from the VTTBOTS feature film.
When Admiral Nelson is in the flying sub over Washington the scene outside his view port is moving horizontally/side-to-side. That could only happen if the ship were flying sideways. The same thing happens again as he comes in for a landing at the airport.
Watch for a camera shadow on the bad-guy Weber right after an irritated admiral says they will "wait at the coordinates and that is all", as Weber and the rest of the committee in the room rises to leave.
To facilitate the plot of the flying sub going to Washington,we are shown the Pentagon next to a body of water that does not exist there-also,with the possibility of the world in demise,there appears to be a motor launch of happy people in bikinis-in a motor launch-waving at the camera without a care in the world.
Adm. Nelson states that the coordinates the Seaview's missile must be launched from is 125° west and 58° south. After he says this, Com. McHenry walks over to a large world map and incorrectly points to a location south of Australia. He apparently confused the 125° east longitude with 125° west, which is not what an experience Naval officer would do.