Goofs

When Keisuke drives to the cave to search for Masahiko, Professor Miyajima and Eiko, he drives there in a fast back coupe. However, when the car explodes it inexplicably changes into a four-door sedan.

The fake Godzilla disguise that MechaGodzilla initially wears makes little sense. MechaGodzilla's shape doesn't match up with his suit; for example he can bend his fingers when disguised, even though it's later revealed that his fingers are stiff, unbendable missiles. He also has a large "crest" on his head, but when wearing the suit, his head is the shape of Godzilla's.

Mechagodzilla is made of "space titanium", presumably nonferrous. Yet after Godzilla becomes magnetic he's able to magnetically tractor Mechagodzilla.

Visible wire on Godzilla's neck right before the pyrotechnics go off and he starts bleeding.

During Godzilla's confrontation with Mechagodzilla at the oil refinery, several flashes of light cause the two monsters to cast shadows against the sky, revealing the sky to be a matte painting.

MechaGodzilla constantly reloads his finger-missiles, even though the missiles are much bigger and wider than the barrels he shoots them out of.

When the alien henchmen revert back to their ape-like forms, the entire screen freezes while the transformation lasts, including other people in the shot. There is an obvious crossfade effect, during which the "frozen" people slightly change position.

When Mechagodzilla is fighting Anguirus, the strings to pick Anguirus can be seen clearly.

How does an old earth prophecy get fulfilled by visiting invading spacemen is never explained.