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.
In The Dark Knight it’s bothered me that two-face didn’t develop any pronunciation problems after losing a cheak and half a pair of lips. I know, I know, it’s a comic book movie, but seeing him pronounce the ‘b’ sound with his teeth showing takes me out of it every time