Star Wars Episode III: Revenge Of The Sith
Mace Windu was one of the most powerful Jedi in the entire galaxy. Even then, it wasn't very smart of him to go face the chancellor by himself. He could have gotten a bunch of different Jedi to help bring the evil villain down, but instead, he went by himself and immediately got wrecked. Seems pretty avoidable!
Mean Girls
Cady Heron was the new girl in school. Yes, she weirdly melded with the plastics, but this plot hole still doesn't make sense: How would she be responsible for the burn book if she just moved there? There was a lot of old tea in that thing that Cady wouldn't have known. She would have at least needed some help!