Harry Potter
and the
Order of the Phoenix

Thestrals

People who have seen someone die can see thestrals. Harry is able to see thestrals, presumably because he saw Cedric die. But why wasn't he able to see them prior to book 5? He saw his parents die (or did he actually see them die? or was he too young to understand?). He saw Quirrel die in book 1 (or did he actually see him die?). He saw Tom Riddle die in book 2 (or did that count since the Tom Riddle he saw die was only a horcrux?).

But one thing for sure — he should have seen the thestrals when he left Hogwarts and the end of book 4. He had already seen Cedric die, but the description in the book clearly has him leaving Hogwarts in the horseless carriages.

House Elves

In book 5, Hermione spends a lot of time making hats and socks to leave lying around in the hope that house elves might pick them up and free themselves.

I don't see how a house elf would be freed by picking up a piece of clothing. Presumably wizarding families use house elves to do all their dirty work — I'd expect that that would including taking care of their laundry. Sounds to me as if the house elf's master would have to actually hand a piece of clothing to the house elf to free it. And certainly no student would be considered to be the master of any of the Hogwart's house elves — only Dumbledore should be able to free a house elf. If just anyone can free a house elf by giving it clothes, Harry could have freed Malfoy's house elf himself by handing the sock to Dobby instead of tricking Malfoy into doing it.