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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Youtube is already taking those videos down though. I tried searching for like 10 minutes and could not find any videos except for 2 or 3 that were really low quality