Harriet's harsh thoughts about her friends in the notebook...
Honestly, I don't even think Harriet was "wrong" for putting those things in the notebook about her friends and classmates.
Whether we want to admit it or, we've ALL had similar, private, terrible thoughts about people around us. It was her private notebook and what's in it is her business. Her biggest mistake was definitely getting careless and leaving the notebook at the park. And that girl was terrible for reading it out loud.
But really, though, I thought the second half of the movie was just a bit too mean-spirited and over the top. Especially when Harriett confronts that one girl (the snob) and says something to the effect of "the reason why your father is away all the time, is because he doesn't love you".
I've never read the book, though, so I can't compare. But the second half of the movie just felt so harsh.