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.
The bonk on the head to incapacitate. It always works instantly and has no long term implications.
Thief (The Dark Project) is one of my favorite video games of all time, but the instant KO blackjack weapon causes one of the hardest splits in play style in any genre of games. Using the magic sleeping stick, it’s fairly trivial to take out all the guards and have free run of the map. Not using it essentially requires some form of “ghosting” (beating the level without being seen at all) which is a significantly tougher challenge in many cases.
The prevalence of the trope certainly speaks to some deep inner cromagnon urge in us for problems to be solved with a simple “bonk!”