When there is a continuity error and/or the plot holes make it seem as though the characters are just completely stupid...
Ex: Character has a powerful weapon/ability/spell, uses it once, never uses it when they need it most AFTER they've used it previously.
When protagonists seem like they're going to lose a battle, but a really long monologue about friendship or something cliche somehow keeps the enemy from not ending them in the middle of their speech and end up winning. And this repeats CONSTANTLY.
I hate it when the main bad guy of a story is beaten by a simple attack such as FWIENDSHIP or some bullshit moral boost from our main hero. I think it's very unrealistic and ruins an entire fight for me. It kills off the entire struggle that the main hero had (if any) trying to fight and kill this guy.