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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I remember glorious porch day and myself originally coming to the forums thinking it will get me Recon armor in halo 3. Came around 2007 and stayed for a couple years then totally forgot about this place. Came back due to interest in destiny saw the shit desticle community and worse game and re-descovered the flood.