I think I read that on Know Your Memes apparently it's written on a maze wall and has come to mean a Reward based insentive where the reward is never forthcoming.
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Wed 20 Apr 2011, 22:49,
archived)
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Wed 20 Apr 2011, 22:53,
archived)
and then there's a scene in the closing titles which suggests there really was cake, but it's left a bit deliberately ambiguous, to toy with your emotions.
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Wed 20 Apr 2011, 22:56,
archived)