Revealing mistake: When Francis emerges from the cave, he stands in the foreground, with the distant mountains and sun in the background. The sun creates a lens flare, which is an internal reflection within the camera lens. Yet Francis is in front of the flare. This shows that the actor was composited with the background via a traveling matte process.
Revealing mistake: During the scene when David is fighting off the large beast in the arena, there is a shot where you are looking right down its throat. Inside the "creature" you can see a light emanating from inside the mouth of the suit, probably so the actor inside could see while acting in it.