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.
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.