A flashback to when Leonard first moved in with Sheldon and they signed the roommate agreement.
Before there was Leonard, there was an even angrier roommate. Angry enough to write "Die Sheldon Die."
A flashback to the nerds when they first met playing Gamecube together.

The Big Bang Theory Season 3 Episode 22 Quotes

Leonard: What am I doing in your spam folder?
Sheldon: I put you there after you forwarded me a picture of a cat playing the piano entitled, "this is funny."

Sheldon: Roommates agree that Friday nights shall be reserved for watching Joss Whedon's brilliant new series, "Firefly."
Leonard: Does that really need to be in the agreement?
Sheldon: Might as well settle it now; it's going to be on for years.