Just an observation - "Snape's Worst Memory" happened at the end of his fifth year, but he used the hexes that were clearly developed (crossouts and everything) in the margins of a sixth-year text. I wonder why he chose to use his mother's text that way - I'm guessing that, since it was a semi-magical household and he showed such great promise in Potions, she allowed him to experiment with the book. Or he snuck away to to do it. And if he were recording potions experiments, he might as well do the hexes, too.
It's also impressive that he developed those hexes at age fifteen.
On another note: Do I have a different text than everyone else? Because in all of my books - and I have several copies of them - Harry's mother's name is spelled Lily, yet I'm seeing Lilly everywhere. And in my books, British witches are called Madam, yet I'm seeing Madame everywhere.
I'm really wondering. Am I the one not seeing what is written?