Can be anything, from characters not using objects they have on them, to physics not being realistic, or a very big plot hole.
As an example, one of my friends told me that his pet peeve is that in a lot of sci-fi movies, when spaceships run out of fuel, they stop moving, while inertia and lack of atmosphere should keep them in motion.
To generalise it a bit, this could be applied to helmets/masks required but not worn in films, war films and tv for example. In dune the main actors barely had their masks on, but the extras could be bundled up cause we don’t need to see their faces