Revealing mistake: When Flash is rescuing Barin from falling into the void, he rubs against one of the spikes and it bends as if made of rubber. (01:12:55)
Revealing mistake: When the Hawkmen city is destroyed, you can see the lower portions of it wave about, revealing that it is in fact a model. (01:20:15)
Revealing mistake: When the Hawkmen start their attack on Ajax, in one shot you can definitely see the wires that support them. (01:27:20)
Revealing mistake: When Flash and Vultan are watching the battle, Flash is on his bike with moving hair and all, but the distance to the shuttle never decreases, revealing he's really not moving. (01:27:35)
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)
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)
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.
Revealing mistake: When the spaceship flies, Zarkov keeps warning about the g-force and Dale and himself suffer it, yet when he tells Flash to step on a pedal, Flash manages to lift his foot and do everything effortlessly, g-force magically gone.
Revealing mistake: When Ming dies, one can see his real left eyebrow. The makeup covering it is missing.
Revealing mistake: Zarkov's rocket blasts through the greenhouse ceiling, but doesn't burn or scorch a single millimeter of the place.
Revealing mistake: While Flash and Barin are fighting with the whips, Flash takes a swipe at Barin with the whip and hits him in the face. Before the whip strikes Barin, you can see the red blood on his face, we then hear the crack of the whip, Barin yanks his head back as if he had been hit and then comes back into frame smearing the red blood.
Revealing mistake: Flash is inside his car reading when hot hail starts to fall in front of the camera. Problem is, the shot is from inside the car, which is absurd.
Revealing mistake: When the movie ends and the Hakwmen say thanks, watch for the people on the far left: they become greenish transparent because of the blue screen.
Revealing mistake: While Dale and Zarkov fly to Arboria against a blue screened backdrop they become transparent.
Revealing mistake: When Flash takes a beating after saving Dale, all the kicks he receives are very poorly performed, most of them never reaching his body.
Revealing mistake: Flash flies on the Cosmo-mobile and half of it becomes transparent due to the blue screen behind.
Revealing mistake: In the plane, when the eclipse occurs, the inside of the plane including Flash and Dale's faces turns to a bright fakish red; an impossible situation that gives away the lighting filter used to dramatize the eclipse effect.
Revealing mistake: When the movie starts, hot hail starts to fall, but in the close-ups it just falls on a tiny area on the ground, in the rest of the wide angles the hail is gone, it neither falls nor is seen on the ground.
Answer: Spider.