Apparently Halo 4 leaked almost a month in advance. Supposedly someone swiped a few discs off the production line as they were being printed, and on other forums, images, videos are being shown.
I won't post any link nor will I discuss it, but this always apparently happens for Microsoft. If it wasn't Halo 4, it was Gears of War 3, and before that Halo:Reach.
I'd advise you folks to go "dark" as the vids are already on youtube and people have already livestreamed the entirety of the campaign, including the endings.
[Edited on 10.13.2012 8:49 AM PDT]
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I don't have as much willpower as others (still remember the Halo 3 endings on Youtube 5 years ago), I saw the first Campaign mission and the closing cutscene. No spoilers here. The game will be awesome (hopefully), don't let anyone ruin that.