Revealing mistake: When Biro stabs the Imperial soldier with his sword, it's obvious that the sword is angled to miss his body. (01:30:50)
Continuity mistake: There are a couple of instances in the movie where the models do not correspond with the actual sets. When Klytus arrives at the Hawkmen city he lands near the top of the city. Yet in that very hall there is a pit with a deep void underneath. Such a room could only exist at the bottom of the city. Also, when the Hawkmen start to land on Ajax, its engine section appears to be missing. (01:10:10 - 01:14:30)
Continuity mistake: Several small meteorites are shown falling to earth. Two of them land in water, but only one of them creates steam. (00:05:10)
Revealing mistake: Just after Klytus has his sticky curtain call, and King Vultan orders the evacuation of the Sky City, watch for the wide shot with the fleeing Hawkmen superimposed in the background. As Vultan tells our heroes he and his men have too much to carry to give them a ride, watch his entire multitude of brave fleeing Hawkmen suddenly disappear one frame before the next shot cuts in.
Continuity mistake: As the Hawkmen evacuate the city, all the furniture is seen neatly assembled. When Ming arrives, however, the furniture is strewn all over the place. The only people who could have moved it would have been Flash's group, and why would they do that? (01:16:10)
Continuity mistake: Klytus is killed when he falls onto three spikes on the platform. In subsequent shots, the spikes that have pierced his body are gone. It can be most seen a minute later into the film, when the platform tilts upwards. (01:15:15)
Continuity mistake: When Flash, Dale and Zarkov first land on Mongo, the shots from inside the ship show Ming's guards walking right up to the ship's window and looking inside. When it then cuts to an outside shot, the guards are much further away.
Continuity mistake: The colour of the lighting shining on the archway wall behind General Kala changes from white to green. (00:41:40)
Revealing mistake: Just after Vultan evacuates War Rocket Ajax leaving Flash on board, there is a shot of the rocket heading towards the camera. There is a green-tinted box surrounding it that the special effects team forgot to remove, after it had been rendered against the background. (01:36:05)
Continuity mistake: When the Barron and Flash are fighting with the whips, they hit each other and cut each other on the arms, hands and face, but in the scenes after the fight, neither one of them have so much as a scar where they were once cut.
Continuity mistake: When Barin and Zarkhov are listening to the Colonel on the video screen, the smoke from the control console appears and disappears between several shots. (01:36:35)
Continuity mistake: When Klytus' ship arrives at the Hawkmen city, it is seen hovering in the sky. In the next shot, it is moving forwards, further away from the city and at a different angle. There is also no engine flare at the rear, which would signify movement. (01:14:00)
Continuity mistake: When Ming is impaled on the nose spike of the Imperial craft, the spike is perfectly horizontal. When he eases himself off the spike, it is pointing downwards much more.