When Godzilla jumps into the water, his appearance changes. This is because this shot is re-used from Godzilla vs. The Sea Monster (1966), which wade produced when the old Godzilla suit was still being used. This film marked the debut of the new suit.
When Jet Jaguar is spinning around and getting dizzy, in the wide shots the Jet Jaguar doll's feet are together and fastened to whatever is spinning him around. In close-ups, the actor in the Jet Jaguar suit is stumbling around like he should be, feet apart.
The car of the main heroes doesn't seem to be pulling any kind of trailer after they flee from the lake, yet the huge water craft that the child had been using at the beginning had to be transported somehow.
At the end of the film, Jet Jaguar walks away from the two young men and the kid, who run after him. Just before they catch up with him, there is a shot in which Jet Jaguar and the three others are standing still, then begin moving again. Obviously the actors were waiting for their cue. Although this goof was fixed in most later video versions, the recent German DVD edition still has it.