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: This film has the old movie trait of Nazis having German accents rather than actually speaking in German, which is fair enough as they take up the majority of the screen time. However, not everyone adheres to this rule and there are many characters, minor and major who slip in and out of accents, and many never have them at all.
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.