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.
It always bugs me when someone stands up, unfolds a few bills and drops them on the table. Then they just walk out. How do you know that’s enough? Maybe someone ordered some 32 oz margarita with top shelf tequila. Are they just leaving a massive tip? Or are they leaving a shitty tip and don’t want to look their server in the eyes?