Goofs

Due to stock footage from earlier movies being used, the appearances of Godzilla, Ghidorah, and Anguirus keep changing.

Due to stock footage from earlier movies being used, the sky often changes from day to night.

When Godzilla is fighting Ghidorah next to the mountain, it is a stock shot from Ghidorah: The Three-Headed Monster (1964). As Ghidorah knocks Godzilla into the bridge, the Mothra larva appears at Godzilla's feet, even though Mothra isn't in the film.

After the Godzilla Tower is destroyed, the shot of Godzilla rising is from Invasion of Astro-Monster (1965). When he gets up, Rodan is visible in the background, even though he never appears in the film, aside from a stock-footage shot used earlier in the film.

Due to stock footage taken from various films, in some shots during Tokyo's destruction, the city background suddenly changes to a grassy area.

It is painfully obvious that in the shots which show Gidorah and Gigan flying, they are stiff dummies. In scenes that take place on the ground, both move around quite a lot, and Gidorah is always seen swinging his three heads about, so the flying scenes stand out like a sore thumb.

Pieces of Godzilla clearly keep falling off during some shots, indicating that the suit was already decaying during filming.

The main human characters, when they descend to the ground on a cable, it is obvious that they are mere miniatures in reality. For one, they don't have the proportions of a human being, nor do their clothes look like the ones the actual humans were wearing.

As Gigan steps on a car while ravaging across town, the front bumper and grill pop off the car, revealing that no engine is inside the plastic hood.

When Gigan crawls behind some trees, it cuts to stock footage taken from the film The War of the Gargantuas (1966), and you can clearly see that instead of Gigan, it shows one of the Gargantuas from that movie hiding behind the trees.

In many shots, Anguirus keeps roaring but he doesn't open his mouth.

When tanks are coming to stop Anguirus, a wire can be seen pulling the tanks forward.

When Anguirus gets his face slashed by Gigan's buzzsaw, you can briefly but very clearly see a gloved hand pushing Anguirus' head towards Gigan by grabbing onto the spikes on the back of his head.

In the English dub, in which the monsters infamously talk, Godzilla mispronounces Angilas' name as "Angila".