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.
My specific example is The Butterfly Effect when he gives himself stigmata in prison; the movie has already established that’s not how it works. Looper made a similar mistake, again not following the rules the movie itself set up.
The more general example: sprinkler systems. Basically every single time a sprinkler system goes off in movies and TV it’s wrong. They get it wrong so consistently that it’s trope at this point and would confuse people if they decided to get it right.
Each sprinkler head is triggered by heat individually, so smoke won’t make the place get flooded out, and holding your lighter up to the head will only make that one head go off. The systems would be so poorly designed to work like in the movies, with false-triggers and flooding areas nowhere near the fire all the time causing more damage than fires.
The water in those sprinklers is disgusting too! It’s been sitting, stagnant in that pipe for years. It won’t be clear water like from the tap and it will stink.
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.
I don’t know about residential sprinklers, but industrial ones will trigger a cascade of sprinklers to flood. The first one drops the pressure on the line, which activates the high volume pump, which will cause all sprinklers heads to open.