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.
Characters in dystopian settings with clean faces with makeup, and perfectly coiffed hair. You’re telling me that whoever built your doomsday vault decided that a 10 year supply of concealer, mascara, lipstick, etc should be part of a survival kit?
What is even the point of living through the apocalypse if you can’t shave your legs!