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.
This one is a bit more specific, but in the new dune movie, literally everyone just starts panting and heavily breathing through their mouth after any exhertion.
The fremen absolutely know better, and I’m almost positive there was a line in the first movie when they were explaining desert survival to Paul where they say you must preserve as much moisture as you possibly can so always breathe through the still suit
They hardly ever wear goggles or even sunglasses either! Their eyes would be ruined instantly.
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