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 wife pointed it out and now I see it everywhere. How hard would it be to just put a glob of playdoh in there or something for the weight?
Or even better come up with liquid filled inserts if they are worried about spills, so you get realistic sloshing movement. They are so concerned with realism in guns in movies that actors get shot by accident but when it comes to cups they’re like “shrug”.
That’s quite reasonable. Like a sippy cup inside.