Revealing mistake: When Sue Snell wakes from her nightmare, listen carefully as her mother comforts her. At one point, she calls her Amy, which is the actress's real name (they're mother and daughter in real life).
Suggested correction: Sue's mom just keeps repeating it's all right, you never hear any name mentioned.
That's because the music is supposed to drown out any dialogue.
Revealing mistake: When Kong first shows up to take Dwan away, he roars and pounds his chest. As he does so, the breastplate in Rick Baker's costume vibrates visibly in a way that a flesh-and-blood chest wouldn't.
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.
Revealing mistake: When Annabelle is skiing at the end (just before she turns into Ellen) we see a shot of her from the back of the boat. Its obvious that she is skiing against a backdrop cause the water is too far down for her to be touching it.