Parseltongue.

Hmmmm.. if you have seen the recent Mel Brook's musical 'The Producers' you will surely recall the hilarious moment when Roger Bart opens the door to Nathan Lane and Mathew Broderick and goes 'yesssssssssssssssssssssssssssssssssssssssssssssss?'. So if Bart just happened to study at Hogwarts, he would probably open the portal to the Chamber of Secrets half a dozen times a day by mistake. Parseltongue cannot be learnt. It is a gift inherent to certain wizards, most of them Dark which enables them to talk to snakes. Assuming that Ron would be able to open the Chamber of Secrets by standing in front of it and hissing is a ridiculous statement and JKR should have come along with something more plausible. And while we are at it, what is the deal with Fiendfyre? Someone should have let us know that it can destroy Horcruxes. Sure, the fact that Basilisk venom can was long known, or atleast indicated. And that fact that Gryfffindor's sword could also destroy a Horcrux, while not directly told to us, was atleast obliquely hinted at.. and there is a pretty huge mythology/literature about special swords with special properties. But Fiendfyre??? Come on, that made it appear that JKR's editors were yelling for a deadline and she just put that in to save pages.