Harry Potter Oddities

While re-reading all of the Harry Potter books over the last month, I've noticed little things here and there that don't quite mesh up. Some of them are not listed here because J.K. Rowling explains them away on her website. Although I did not do a thorough search there for the ones below, I don't think they've been addressed:

Book 2: Percy docks points from Griffindor when Ron and Harry exit a girls bathroom (Moaning Myrtle's)
Book 5: Hermione points out to Malfoy that prefects can't take points away from houses

Book ?(multiple): Harry references when he first met Draco on the Hogwarts train
Book 1: Harry first meets Draco at the robe store

Book 6: The library fails Hermione for "the first time" when looking for Horcruxes
Book 4: The library actually fails Hermione for the first time when trying to figure out how Harry can breathe under water

I'm not quite done re-reading Book 6, so I will edit this post (with a note) if I realize any more oddities.