Plot device
Chekhov's gun
The principle that a conspicuous detail introduced early must become significant later; nothing is shown without purpose.
Chekhov's gun in fiction: 4 works
Ice-nine, introduced casually, becomes the world-ending device it was always destined to be.
Early warnings that 'life finds a way' and the failsafe systems pay off exactly as foreshadowed.
The spinning-top totem, planted early, becomes the film's final decisive image.
The daughter's malfunctioning hearing aid, established early, becomes the creatures' fatal weakness.