The servers will crash and you will spend the entire day groping your game case and most likely doing inappropriate things to it since you won't be able to play the game.
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#Destiny
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Honestly I think Bungie will be decently prepared. They did stress tests and closed off servers during the beta, smushing as many players into as few servers as they could. I think they'll have a handle on launch day, not that some issues might pop, but I bet they will be minimal.