An example that comes to mind: why did Harry need a permission slip to go to Hogsmeade? It had already been established that parental consent meant fuck-all seeing as how his guardians didn't even consent to him going to the school in the first place. But his restriction from going to town was necessary to advance the plot, so she wrote it in. It's just lazy writing. And there are countless examples of inconsistencies like this.
Like I said, she deserves all her success and I'll admit, I enjoy the series. But it's held to a ridiculous high esteem by some of the hardcore fans. And that it doesn't deserve.