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.
Continuity mistake: When Dale escapes and fights the guards, the editing is really awful: first she knocks a guard, who later on changes positions. Then she walks away, leaves her shoes on the floor, and hides against a wall, but after a brusque cut she appears somewhere else, the shoes are 15 meters away and the guard from before lies in front. (I guess that some of the fighting in between was deleted.)
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.
Continuity mistake: During the plane flight, the sky outside keeps switching back and forth between cloudy or puffy clouds opening to let a red light, depending on the angle.
Continuity mistake: The hot hail that falls on the hood of Flash's car disappears in the later shots.
Revealing mistake: While Dale and Zarkov fly to Arboria against a blue screened backdrop they become transparent.
Continuity mistake: When the plane enters the greenhouse the cabin is empty. Then in the close-up Flash and Dale have appeared inside.
Continuity mistake: When Ming walks in the room where Dale sleeps, her thigh swaps from covered to uncovered and the cushion moves around depending on the angle.
Continuity mistake: After the meteorite on Munson's bed starts burning, his hand moves from down to up and is now holding a pen and paper, despite him being asleep. Also the cup on the table moves around between shots.
Continuity mistake: When Flash greets the martians he's handed a gun which makes him crouch and twist to the left. When the angle changes he is now standing tall and his hand is at chest level instead of waist level.
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.
Continuity mistake: When Ming orders to send another charge against Sky City the sky swaps from sunny in the wide angles, to cloudy in the close-ups in barely half a second.
Continuity mistake: When the laser scans Zarkov's jacket to destroy the gun inside, the arm disappears in the close-up angle.
Revealing mistake: Flash flies on the Cosmo-mobile and half of it becomes transparent due to the blue screen behind.
Plot hole: A group of baddies in Ming City fire at Flash Gordon from one laser gun while he drives his jet, but from the opposite angle the shots come from everywhere, as if there were dozens of guns.
Continuity mistake: Ming kicks a silver cup into the void. When it falls, the cup becomes green coloured.
Continuity mistake: When lizard man is fried, the position of his tail keeps changing between angles.
Answer: Spider.