Hermione didn't keep many secrets. But this was one she didn't regret. One shot during Deathly Halloween.

You remember being taught about drug addiction years ago, before Hogwarts. You wonder, as you dive into yet another pensieve, if this is what it's like.

"Lily Potter. I must admit, I never thought I'd be lecturing you." Lily stared at her sullenly, refusing to respond. Words were too good to be wasted on this. She was being lectured by a professor whose class she didn't even take, who would be trying to tell her that she was a murderer for being interested in the Dark Arts.