Mistakes....

okay i am jst re-reading the HP series and i've noticed a few mistakes...

at the end of HP and the prisoner of azkaban when they are just leaving school, it say "and even fred and george managed to get a handful of owls" but when ron and harry get theirs, they have to wait until near the end of the summer holidays where their results arrive by post.

and then in the 4th book, miss fawcett is said to be n hufflepuff when she is in the hospital wing then in ravenclaw when she is caught by snape after the yule ball.

are these actual mistakes?
has anyone found any others?