For me it's always the unexplained power nerfing that authors do just to advance the plot.

Harry Potter in the first 3 books was fearless, he literally took on voldemort with his bare hands.

Then when the dumbass plan with the port key cup happens, he just stands there like an idiot as the rat dude kills Cedric and revives Voldemort as if both he and Cedric don't have wands that allow them to cast spells.

I mean they could have maybe had like 20 wizards camping the graveyard to make escaping impossible, but nah they really tried to make the coward rat guy seem like he was now somehow more capable than all of voldemort's previously defeated plans combined.