The biggest mistake in 'Harry Potter', in my opinion, was killing off Fred Weasley. It felt unnecessarily cruel, not just to the characters but to the readers who'd grown up loving the twins' antics. Fred and George were the heart of the series' humor, a reminder that even in dark times, joy could exist. Losing one of them shattered that balance permanently. It didn't serve the plot in a meaningful way—George's grief was barely explored, and it didn't change the trajectory of the final battle. It just...hurt. Rowling has said she regrets it too, which makes it sting even more.
Another misstep was the handling of Slytherin house. By the end, it felt like a caricature of evil, with no redemption arcs for any student except Slughorn. Even Draco's 'change' was underwritten. It reinforced a binary 'good vs. bad' dynamic that didn’t match the series’ earlier nuance. Imagine if a Slytherin had stayed to fight in the Battle of Hogwarts—what a powerful moment that could’ve been. Instead, they were all evacuated, which felt like a missed opportunity to complicate the story’s moral landscape.