Ahead of his time?

[Jimenem]

The way I see it, just because you learn something one year doesn't mean you can't wait a few months before you write it down. It could have been in his 5th year potions book too, we've neevr seen that. Maybe he re-copied a few old notes into his new book.

Either way JKR's not perfect, no one is. But if little plot glitches bother you that badly, it's easy to make up a possibility for their existence.

[Rory]

In DH, when the trio are in the vault, Hermione uses levacorpus to hoist Harry up to get the cup. HARRY UN-jinxes HIMSELF with Libracorpus. I don't recall any other instance in the books where anyone who was jinxed - jelly legs, whatever, was able to un-jinx themselves. Maybe that was Snape's big discovery. A counter spell you could use on yourself?