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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Sad part is, people do it on purpose to... I remember one time seeing a video where some people actually read the ending for Harry Potter first, waited in their car, and then shouted spoilers at poor kids and girls standing in line as they drove by. I had Halo:Reach ruined for me because I wasn't smart enough to avoid YouTube at all. People purposely write the wrong headings, etc. Stay completely dark for the next few weeks. Heck, I would assume all of us here at Bungie are now used to the "dark" lol