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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Getting banned for telling someone to shut up because they were asking about Bungie Day activities on July 8th, 2007. Friggin idiot, 7/7/07 was the best Bungie Day.