Grant is introduced as the man who retrieved Benes and a frogman in "the war." However, Grant, played by Stephen Boyd was only 8 years old at the beginning of WWII. He would have been closer the the correct age for Korea, however, the Korean Conflict is generally referred to as "Korea" just as a later conflict was just called "Vietnam."
Two times, people go swimming outside of the Proteus. The first is when Grant, Cora, Duval, and Michaels all go to get air for refueling their air supply. The second is when they go to remove the reticular fibers from the sub's airvents. In both cases, people are seen getting in the tank, the tank filling with water, and then they go outside to do their jobs. Interestingly, in both occurrences, they all come back with perfect hairdos (particularly Cora), with no indication of having gone swimming, like one would from swimming in a pool or in the ocean. (Dr. Michaels could be exempt from this, however, as he is bald.)
After the Proteus has finished Phase 1 shrinking and is placed in a test tube, it submerges and descends deep into the tube. During the descent there are several shots looking up past the commander toward the surface. Many bubbles can be seen rising and striking the surface a short distance above the sub. In each upward shot the distance to the surface never changes, even though the sub is shown to be descending rapidly in other shots.
After the opening credits, Stephen Boyd travels to the shrinking centre in a black Chrysler Imperial, but in the long shot, stock footage of a back Lincoln Continental is used driving along a street. He then arrives in the Imperial.
The patient's red blood cells, shown in the various scenes of the miniaturized submarine traveling through blood vessels, are nucleated, which is incorrect. Mammalian, and therefore human, red blood cells have no nuclei; nucleated red blood cells are found in most other animals, however, such as birds, reptiles, amphibians, fish, etc.
The amount of radioactive material for the sub would not need a lead carrying case. Grant proves this by removing the container from the case with no protection and handing it to Owens who inserts it into the reactor, again bare-handed.
The scuba equipment has a low-pressure hose running between the tank and the face-mask. The face-mask appears to have a regulator built in (the silver disk). The regulator reduces the pressure from the tank to a pressure the diver can breathe. The typical pressure of an open SCUBA system with a regulator tank is 200-300 atmospheres. This would blow out the hose as soon as the air was turned on.
Antibodies attack Cora, even though Benes' immune system would never have encountered her before. Antibodies are highly specific to what they bind to, and generally require 5 to 7 days to form against something the immune system has not encountered before.
When the crew of the Proteus are refilling their oxygen, they are using a source that has molecules almost equivalent in size to themselves, while the oxygen that was stored on the Proteus prior to reduction would be smaller in relation to themselves - the source oxygen molecules could not physically fit into their tanks.
At the end, when the "micronauts" escape Benes' body through the tear ducts, there are just the four of them that make it out. As they left the sub and the fifth member of their crew still in the body (their mass admittedly "digested" by a white blood cell but still all within Benes' body) then the body (and perhaps the entire operating room) would be destroyed by that mass returning to its full size - thus killing Benes, whom they were intending to save. (Asimov's novelization corrects this.) Also, the saline solution that held the sub was also miniaturized, so logically it should have expanded as well.
The crewmen reported before the sterilization procedure that Benes' heart rate was 32 beats per minute and it was his respiration that was 6 breaths a minute. But even so, these heart rates were still inconsistent.
A slice of Capt. Bill Owens' right side of head is missing in the scene in the vein after the arterial-venous fistula incident, probably due to a mismatched matte. (Approx 44 minutes, 57 seconds into the film)
As the car disappears down the lift, delivering Grant to CMDF, it should vanish as it is hidden by the hole. Instead the special effects crew elected to shrink the image; we see at least 20 feet of elevation change at the back wall but the car is still visible at foreground.
When the shrunk vessel submerges in the liquid, there are interior shots showing a lot of very small bubbles around the vessel. The depiction of the fluid flow is incorrect. There should be no such small bubbles when such a small object dives as the adhesive forces within the liquid would prevent that. Compared to the shrunk vessel, the bubbles should appear much bigger.