You guys are missing something here. How did Leonard manage to avoid seeing the movies and also internet chatter before finally reading the book for that even to still be a spoiler?
Hell, I'm not a Potter fan, but I've at least seen the movies and read online all the major plot points without having opened the books.
I personally found it strange that he didn't know the spoiler already. It's just one of the many details the writers get wrong on the show when dealing with "nerd culture".