As Seen In: Alien (1979)
You know a special effect is dynamite when the actors on set freak out worse than any audience member ever could. That's exactly what happened during the filming of Alien's most infamous sequence, in which actor John Hurt's stomach gives birth to a little "chestburster" alien.
Considered one of the greatest shocks in cinematic history, director Ridley Scott's deathblow was designed behind-the-scenes as a surprise to Hurt's in-scene co-stars, particularly Veronica Cartwright. To achieve the effect, Hurt was positioned underneath the table, with his head and arms sticking out, and the fake stomach was rigged with bloody makeup and red-spewing squibs; only, none of the other actors knew about the impending blood geysers—they just thought that a freaky puppet was about to jump out.
So once the chestburster popped up to say hello, the splashes of stage blood jolted the performers, the wet blast to Cartwright's face causing her to scream—a genuine reaction visible in the film.