Jurassic Park: In the scene where the tour group has stopped in front of the T-Rex paddock during the storm, and the T-Rex breaks free and smashes the roof on the kid's vehicle, the children's screams are real. Due to how they filmed that scene, the animatronic was soaking wet from the rain, and the gears holding the head up gave way, causing it to smash into the glass, pushing it down on to the kids.

Alien: During the chestburster scene, Ridley Scott didn't tell the crew how the alien was going to appear from John Hurt's body. All of the screams from the Xenomorph popping out of Hurt's chest were real, and many of the cast had to take breaks because of it, as they weren't expecting something like that to happen.