A dead diver is shown floating face-down in the ocean. He has twin air tanks on his back, but when the body is pulled up into a boat, his gear has changed to a single tank.
At about 0:47:30 into the film, the words "U.S. Navy" are stenciled on the back of an enlisted British sailor.
When the British aircraft carrier HMS Royal Oak is sent to confront Gorgo's mother, the stock footage of jets taking off from a carrier show U.S. aircraft.
When they are about to set fire to fuel dumped in the River Thames as Gorgo's mother wades up it, the wreckage of Tower Bridge behind her can be clearly seen in one shot, but the bridge is not destroyed by her until a few minutes after the fuel is set on fire.
When the ship lowers a bathysphere to look for Gorgo, the captain says the depth is 10 fathoms, which is 60 feet, but the height of Gorgo is later stated to be 65 feet. Gorgo was five feet taller than the water he was in.
The sailors put into port on what is supposed to be a small island off the west coast of Ireland. They encounter a local man and ask him a question, which he answers in Irish (Gaelic): "Cad a dhéanfaimid anois?" while gesturing in the direction they need to travel. This means "What will we do now?" Anybody who understood this phrase will wonder why he would answer in this way until later in the film, when it gradually emerges that this is what the actor is supposed to say when required to utter a line that the audience is not supposed to understand. There is one occasion in the film where the phrase actually fits the situation.
When the current is turned up to try to electrocute Gorgo's mother, the dial gauge shown measures Pressure In Pounds Per Inch, not Electrical Voltage.
The radio reporter in the film refers to Tower Bridge as one of London's oldest landmarks. In fact, it was barely 65 years between its opening and the film's release. The Tower of London is close by, approximately 850 years older, but much less spectacular to destroy.
When Gorgo attacks the bathysphere, he roars at it. Underwater. If the monster actually tried to roar underwater, a torrent of bubbles would come out of his mouth, but no clear roaring sound.
The Admiralty orders an aircraft carrier to the vicinity of Nara Island when all communication with the island is lost. The planes seen taking off are American ones.
When Joe and Sam take off in pursuit of Sean, their car is shown as a left-hand drive, when in actuality, the cars in England are right-hand drive (the film is reversed).
As the ship carrying Gorgo arrives in London, there is a very dodgy effects shot of the real Tower Bridge in the background and a painting of the Dorkin Circus in the foreground, including motionless painted people standing rather too close to the camera to be truly convincing.
A scene in the film in which a London bus displays an advertisement for DULUX paint is also reversed, as XULUD.
As Gorgo is captured and lifted out of the sea and onto the ship, some parts of the net are transparent due to the blue screen effect.
Virtually all the planes seen in the film are American, as is the submarine crew, who are wearing leather bomber jackets that British sailors would not wear.
Shots of Gorgo's mother among buildings and people in the film are doubled.
The aircraft carrier footage was from USS Oriskany (CVA 34) not a British carrier.
When the British radio operator relays the message from HMS Royal Oak to Admiral Brooks, the film's audio dialogue does not match what he is actually saying.
When the "flame throwers" are used on Gorgo, you can see the gas line running down the inside man's pants legs and trailing behind them.
Three bare-headed youths are on a flight of steps leading into the River Thames watching Gorgo's mother coming up the river. A flare is shot into some petrol on the river. Flames rush over the water to the steps, the youths scream and one rushes up the steps on fire, but he is wearing a cap and is obviously an older man.
The older man wearing a cap is standing behind the three young men on the steps when they are watching Gorgo's mother.