The Huge Magical Snake Problem

Harry Potter is having an absolutely brilliant summer at the Dursley's - as usual. His aunt and uncle seemed to be especially nasty to him, and the Dark Lord's nightly meetings haunt his dreams (due to the fact that he never truly recieved satisfactory Occlumency training). One night, Harry witnesses Voldemort make a huge mistake that would effect the rest of his life. NO SLASH.
[This is Severitus (almost), and past the 5th book, OoTP, I'm just throwing canonicity out the proverbial window.]