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.
Can we talk about that scene in The Office where Michael proposes to Holly, and sets off the sprinklers and everybody just shrugs it off and it is never spoken about? It’s a paper company. Everything would be ruined.