The spring-loaded cat is a staple of poorly-written horror movies (some cheap, some not). Typically, the music will slowly swell, creating tension, then a cat will jump out of nowhere and scare the hell out of characters and spectators alike.
The term (supposedly coined by Joe Kane, "the Phantom of the Movies") can be applied more generally to any sudden false scare in a movie.
The most original variation on this theme must be the spring-loaded Jesus from Clive Barker's movie Hellraiser. During a tense stalking scene, a scared girl bumps against an armoire of some kind and a life-size statue of Jesus half-falls out of it, looking ominous and symbolic, as if somebody had placed it there for just such a moment. (How is this symbolic, you ask? It's open to interpretation. You could take it as an icon of suffering and sacrifice, or make your own "Jesus coming out of the closet" joke).