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.
At the Earth's Core (1976)
1 revealing mistake - chronological order
Directed by: Kevin Connor
Starring: Peter Cushing, Caroline Munro, Doug McClure, Cy Grant
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.
Dr. Abner Perry: Oh, they're so excitable, like all foreigners.