Revealing mistake: Flash flies on the Cosmo-mobile and half of it becomes transparent due to the blue screen behind.
Sacha
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.
Sacha Continuity mistake: When lizard man is fried, the position of his tail keeps changing between angles.
Sacha Continuity mistake: After Barin stands up on the spiked ring his whip keeps changing positions depending on the angle.
Sacha Continuity mistake: While fighting against Barin, Flash's whip lies on the floor and keeps changing from rolled to straight between shots.
Sacha Continuity mistake: When Zarkov is waving a piece of cloth to signal Ming their position, Flash and Barin are standing behind, but disappear when Ming watches Zarkov on his screen, and reappear afterwards.
Sacha Continuity mistake: After Barin saves Flash from the Arborian beast Flash's shirt is clean, a frame later from a different angle it's covered in mud.
Sacha Continuity mistake: When Aura is kissing Flash and telling him about her palace, she has her arms around his ears. When the angle changes the hands are around his cheeks.
Sacha Visible crew/equipment: When Flash is attacked by the alien he falls on a noticeable brown safety mat.
Sacha Continuity mistake: Zarkov tells Dale to press the button to launch the shuttle, and Flash leaves the place with a big cloud of smoke behind. Half a second later, from a different angle, the smoke is gone.
Sacha Continuity mistake: When Flash's plane lands in Zarkov's lab the amount of leaves on the windshield increase dramatically between one frame and the other.
Sacha Continuity mistake: When the meteorite falls on Munson's bed the glasses and papers on the blankets keep changing positions between angles.
Sacha 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.
Sacha Continuity mistake: The first time we see Flash waiting to ride in the plane he has no sports bag next to him, but the next shot he does, and in the wide outside angles of his car the bag is nowhere to be seen.
Sacha 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.
luchadorRevealing 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)
Sacha 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.
Sacha Revealing mistake: After Ming is impaled, he slides away off the peak but there are no remains of blood on it.
Sacha 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.)
Sacha 
Chosen answer: Prince Barin was the fiance of Princess Aura, who took a shine to Flash. He was quite simply jealous.
Ioreth