The first occurrence of the Har Har phenomenon took place in the thread "Snape's Dungeon Office". The exchange went as such:
Magda von Waldemann: "I didn't sink hyou like anysink, Sivaroose," she teased.
Severus Snape: "Har har. Well, I like you, don't I?" he said, ever so smugly.
Immediately, spurred along by most of the current Sueauthors, "Har har" gained popularity as Snape's signature phrase on the Verse. Megan went so far as to spread it to the LiveJournal community capslock_hp, where it enjoyed a brief period as a fad (mostly via badly edited photos of Alan Rickman).
Har Har made a return in "The Paris Apartment", during Anthony Crowley's confrontation with Snape, this time with a "frickedy" inserted in between.