Mine would probably have to be the whole Kitchen 8 thing back when this forum was still called "The Flood".
Next best guess would be when the option to nuke the forum came up from the admins because we were naughty. There was a stick at the top of the forum (I think posted by Achronos) for a long while.
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The first Most Disgusting Things That Happened in School thread that was made in August 2010. Some of those stories are hilarious but some are deeply disturbing, especially the herpes story.