The toys from Toy Story never reveal to humans that they are real if they can help it. Whenever a human walks into the room, the toys play dead. For some reason, Buzz does the same thing, despite him being so positive that he's not a toy. What?
Jurassic Park
When the viewers and characters are first introduced to the ever-terrifying T-Rex, they could hear it a mile away. Naturally, it only makes sense that the gigantic dinosaur would make the ground rumble as it approaches. But then, for the sake of the story, the directors completely change that up.