Harry Potter mistake

In Harry Potter 7, they say Harry Potter could not get attacked by spells when under the Invincibility Cloak, but in Harry Potter 5, he was stunned by Malfoy beneath the cloak!

Thank you for that semi-spoiler.

Sorry, but it is such a little spoiler that it should not matter...
Anyways, arent there someone that could clean this one out for me? I dont think Rowling has made any mistakes...

seeing as Rowlings books are a mixture of other fantasy´s and books thrown together in one, its no surprise.